Wow, somebody really screwed up. The Flood has been released and now you're being ordered around by some AI to go kill a bunch of stuff for reasons you don't entirely understand. Sound familiar? Anyway, this level is full of Flood. Wave after endless wave of them. Soon you will know just how aptly they were named.
There are many reasons to dislike 343 Guilty Spark. The fact that he takes away your shotgun is one of my favorites. You should remember how to use the assault rifle on the Flood though. It's not that bad. And you do get a pistol, which you'll basically be keeping throughout the entire level.
Move forward past the two big vents in the wall and look back behind you to your left. Tucked away in the corner is the first you'll see of a new form of the Flood. This is the carrier form. Following in a great Bungie tradition, this enemy's only attack is to hobble towards you and explode. They release several infection Flood when they do this as well. So if you're anywhere nearby not only do you get your shields knocked out by the explosion, but you'll also immediately get reamed by the infection forms. Fortunately they're really slow. So not only can you get away from them pretty easily, but you can also lead their friends into the explosion when they do go off. Two rounds from the pistol will detonate them quite nicely. So whenever you see a whole mess of combat forms near a carrier, put a couple of bullets into him and BOOM! He takes out all his pals for you. If you want to save the ammo you can just run right past them as well. They flop over and start to blow as you get close, but by the time they actually explode you've run past them. Running up and jumping over them works well too. One thing that does not work well is blowing them up with a grenade. That tends to send them flying around the room like little bomblets. One will inevitably come down right next to you and explode.
Take out this first carrier however you want and let the little guys pop themselves on your shield. Melee as many as you can and run around to let your shield recharge as necessary.
There are two pairs of carrier Flood hiding around the corners ahead so be careful of running away from one and backing into another. Take these out however you want. Four carriers shouldn't be a problem if no one else is around.
Get your pistol ready and move ahead. There are two vents in the wall on the left past the previous carriers. When you walk near them, a squad of angry Flood will drop down. Inch forward until you see a signature on your motion tracker then back up and get ready for them. Target the carrier Flood and take them out as soon as they drop down. Hopefully you'll catch a few combat forms in the explosion. Empty the rest of the pistol clip as you're retreating and back up around a corner so the Flood have to come all the way around the corner to shoot at you. Get out your assault rifle and look for them approaching on your motion tracker. As soon as you see a body part, toss a grenade at the corner and start firing. Back up as they get closer and keep burning through rifle rounds. As you're backing up, swing around so that your back is towards the beginning of the level so you don't get stuck in a corner. Retreat all the way to the beginning if you have to. Once they stop coming move ahead and look for any carriers hobbling along. Here's a big hint for the rest of the level. Don't pop carrier Flood when they're standing in a pile of bodies. First of all, they'll scatter the weapons all over the place making it nearly impossible to find them when you need them. And secondly, they'll detonate any grenades nearby often catching you in a massive chain reaction. So if you see a carrier Flood coming towards you, look at the ground to be sure he has lots of space to explode before rupturing him.
So anyway, lure any leftover carriers into an open space and run by them or shoot them if you wish. Now that the area is secure, scavenge for a shotgun. Even if it only has a few rounds of ammo, it's better than your assault rifle. You may need to turn on your flashlight to scavenge well in here. Oh, and look for grenades and pistol ammo too of course.
Move along past the vents those last guys came out of and look ahead. Multiple waves of Flood are going to come out of the next set of vents. The first group is already there. Several combat forms are waiting to ambush you just on the other side of the sloping walls with the lights you have to walk between. Peek in between them and snipe anybody on the other side. Eventually you'll probably get the attention of the Flood hiding on the other side of the wall, so be careful. When they come, back up, wrap them around a corner, drop a grenade, and blast them with your shotgun. Now you can go back to the rest of the waves coming out of the vents.
Use the same strategy you did with the last group that dropped out of the vents. Target carriers near combat forms first and use the pistol until they start to get close. Then back up around a corner and wait for them. Drop a grenade and open up with the shotgun as they come around the corner after you. This simple strategy will get you through most of the encounters on this level. After each wave is destroyed, take a moment to scavenge and then walk up to the vents to set off the next wave. It is worth noting that the number of waves seems to vary somewhat. Sometimes they'll all come in just a couple of big groups. Other times they trickle out in several smaller waves.
Once you can safely walk around under the vents without triggering more Flood, the area is secure. Make one last sweep through the place looking for goodies then move ahead. You'll be going through a tunnel that leads to the atrium that houses the index. Use tunnels like this one as bottlenecks to concentrate the Flood in small spaces. They're much easier to deal with if they're only coming from one direction a few at a time. Move through the tunnel and get the attention of any combat forms ahead. Duck back into the tunnel and give them a face full of buckshot if they pursue you. Once they're gone, stay near the tunnel and snipe the carriers. Now you have a whole load of infection forms to deal with. You could just run around trying to melee them, recharging as necessary, but there's a quicker way. Pick up any old weapon lying around and use that on them. You'll want to conserve your pistol and shotgun ammo, but that doesn't mean you can't pick up a plasma pistol or assault rifle and use that to hose them down. Once they're mostly gone, toss whatever you used on them and pick back up your real weapon.
The way out of this chamber is another tunnel to the left. If you ever get lost or don't know which way to go, look for the Monitor flying around. He glows bright blue and will lead you in the right direction. Here's another tunnel for you. Move through it and a combat Flood will come leaping at you. Stop and put a shotgun shell into him as soon as he lands. Drop a grenade right on top of his corpse and back up. The grenade should take out any Flood that follow. On the other side of the tunnel straight ahead are more vents in the wall. This time several waves will be coming down. To activate a wave, you have to move forward to around the spot where you wasted that first Flood who jumped at you. So move ahead until you see Flood dropping down then retreat into the chamber with the index. Whip out your pistol, zoom in looking down the tunnel and snipe at any carrier Flood near other Flood. After a few shots, the rest of the wave will come running down the tunnel to get to you. Switch to your shotgun and let them have it. Drop grenades when necessary. After each wave, scavenge what you can, reload everything, and then do it again. After a whole lot of waves, they will stop coming. Move in and thoroughly scavenge the area. You should be doing pretty well by now on shotgun shells, pistol ammo, and grenades.
The hall ahead has another wave of Flood in store for you. Move along and you'll see an infected marine leaping towards you. Immediately start in on him with your pistol. From now on, I'm not going to always explicitly mention that you should target carrier Flood when they're near combat forms. This should just always be a priority for the rest of the game. Anyway, keep backing up and firing until they start to get close. Back up through the tunnel into the index chamber and wait for them. When they come into the tunnel after you, drop a grenade and start firing. After they've been reduced to snot, move ahead. There will be one last combat Flood up on the sloping wall thingy on the right. Snipe him down with your pistol then head over to the giant door on the left.
Wait around by the door for the Monitor to return. You can continue down the hall if you want, but the only thing you'll find is a hoard of infection forms and nothing useful. So unless you just really like to watch them pop, I wouldn't bother.
Move through the door as it opens and you'll come to another huge hall. Here you have a choice. The Monitor is going right and that's the way ahead. But there's a health pack to the left, though it's guarded by about a hundred infection Flood. If you're really low on health, you should probably get the health pack before moving on. It's tucked away in the back corner as far away as possible. You'll activate a whole slew of the little guys, so lead them back to where you were and then pop them however you want. Grab a spare weapon, melee them, whatever. If you're in the blue, don't bother.
Moving along you'll find some more Flood to kill. Move slowly ahead and keep an eye on the motion tracker. As soon as you register movement, retreat. Keep your pistol up and start shooting Flood as they land. Move around a corner and blast them as they come around for you. Give the carriers some space then kill them off too. Same tactics you've been using.
The next battle is a trap. Move ahead past the previous skirmish and stay frosty. As soon as you see blips on your tracker, turn around and run back towards the big door. Flood will be dropping down in front of and behind you. Blast through the ones in your way with your shotgun then turn around and use your old tactics on the others. Pistol, corner, shotgun. You know the routine. And as always, remember to scavenge thoroughly.
Ahead the path will take a turn to the left. Many waves of Flood will come dropping down from vents high up in the walls here, so be careful. As soon as you see them, run away. Don't engage them from around the closest corner. You'll kill them off more quickly, but stand a greater chance of getting overwhelmed. Get a bit of distance on them then get their attention with your pistol. Apply standard Flood-killing tactics retreating as necessary. Move back up to set off the next wave. Repeat until no more come down to play with you.
Now you can safely take that turn to the left. At the end of this hall is another turn to the left, but before you get there a small wave of Flood will come out. Nothing you can't handle. Just back up when you see them coming and gun them down with your pistol.
Move past the bodies and the path will take you down a long hall. You will get a checkpoint then the Monitor will fly away and head through a vent at the end of the hall. One big group of Flood is about to come out of that vent. Another group will come from around the corner ahead on the right. Back up as they drop down and toss a grenade into the middle of either group. Keep nailing them with the pistol and toss another grenade if necessary. Back up around the corner if any combat Flood get through and waste them with your shotgun. Mop up the carriers and infection Flood and scavenge.
Just around the corner ahead is a health pack. As you move forward though three combat Flood will come leaping at you. Put a shotgun round into each of them as you back up around the corner. Three or four more combat Flood will soon come around following the previous ones, so put them each down in turn.
Move ahead now and grab the health pack if you need it. Down the nearby ramp is an over shield. You might want to hold off on grabbing it. The next battle will hopefully be very easy. Move into the next section down the hall. Several pillars and structures, but no bad guys. The section ahead has a little hallway off to the left. It's full of Flood. Slowly work your way around until you can just barely see a bit of Flood down there without setting them off. Bounce a frag grenade or two right into the middle of them. After they've gone off, move into this section and you'll trigger some carrier Flood to drop in from on your right. Any survivors from the grenades will come out as well. Take them down with your pistol/shotgun combo as usual.
If you haven't grabbed the over shield already, do so now. Move ahead past the previous fight until you see Flood dropping down far ahead in front of you. They are the first of many, but ignore them for now. This is another trap. Several carriers will appear behind you, so turn around and be ready for them. Move back towards where the over shield was and take each carrier out from a safe distance. Get rid of any infection forms quickly. You can probably afford to spare a few rounds on them by now. Now turn your attention to those guys that dropped down ahead of you. If they followed you back here, take them out with the shotgun as soon as possible. Otherwise, snipe at them from a manageable distance. They will be followed by many more waves of Flood all dropping down from the same vents at the end of the hall. Slowly whittle them down using the same tactics you've grown accustomed too. You will burn through a lot of ammunition, but eventually they will stop coming. Wade through the pile of bodies and restock your supplies.
Move ahead to the end of the hall and peek in all the nooks and crannies. There is a small group of carrier Flood here and they can sneak up on you if you're not careful. Take them out however you want and dispose of the infection forms.
You have one last fight ahead before you're done with the first floor. And you get some help this time. Move along and a large group of Flood will come down the hall after you while a group of the Monitor's Sentinels move in from the side to help. Plant a few grenades right in the middle of the incoming Flood then duck back into the area the Sentinels came from for cover. Hopefully the Sentinels will take out the rest of the Flood for you. Once the battle stops, go see who won. If there are Flood left, they should be easy prey for you by now. Scavenge through the bodies thoroughly then move ahead onto the lift to the second floor.
Wait, It Gets Worse!
Step off the elevator and move ahead into the hall. Several carrier Flood will come out of the vent to your right, so watch for them. Retreat towards the elevator and pop a few caps in their ass. Mop up the infection forms. Now you're free to cruise down the hall. You'll pick up a new batch of Sentinels along the way.
At the end of the hall the path takes a turn to the left. You'll come across several carrier forms wandering around after the turn. Let the Sentinels take care of them while you take a look around. There are two health packs down a ramp in this first area. You can grab one now if you need it. Move ahead until the Monitor leaves and you get the message to hold your position. Immediately turn around and hurry back towards the area with the two health packs. This is another trap. If you move ahead now a whole ton of Flood will come out of the woodwork and put the smack down on you. Better to stay back while the Monitor does his work. Once he comes back and opens the big door, you can move ahead safely. The massive horde of Flood won't show up. Of course if you want to fight them, you can do so quite effectively using the same tactics you honed on the pervious groups of Flood.
Either way eventually the Monitor will open the door ahead and you'll have another fight on your hands. The Sentinels will move through without you and might even start blasting at a group of distant Flood. You have two options now. You can run ahead and take a shortcut, or you can follow the intended path and face three more battles with the Flood. I'll cover both options starting with the shortcut.
If you want an easier time, run through the giant doorway past the Sentinels and right up to the edge overlooking the central chamber. Turn ninety degrees to your right and you will see a short, sloped wall. Run up and jump on top of it. It's high, but you can get up there. Hop down on the other side and continue along this little inner path around the central chamber. This same shortcut will work on the third floor when you have to get to the other side of the chamber up there. Keep on hopping until you can get back to the outer hall on your right. There's another huge door here, so walk up to it and the Monitor will pull his usual trick of making you wait for him to open it.
If you want to fight your way ahead, move up to help the Sentinels as quickly as possible. Notice the health pack and grenades just before the door on the way. Zoom in and snipe until they get close enough to start battling the Sentinels. It might be a little hard to aim accurately in all the confusion, but toss a grenade or two at the main group of Flood and empty a few pistol clips into the mob until they thin out or start coming for you. Then you can fall back and take them out with your shotgun as they get close. Hopefully most of the Sentinels survived.
Scavenge from the fallen Flood then move ahead and zoom in on the path ahead with your pistol. More Flood will come into view and start moving towards you. Shoot as quickly and accurately as you can. Definitely look for carriers to pop in the group. Once they get close they tend to bunch up and start firing at you and the Sentinels. Toss a grenade into the pack and keep firing. Once it goes off, run in and clean house with your shotgun. Scavenge and move ahead.
The next group includes one of the most despised enemies in the game. A Flood with a rocket launcher. These guys are deadly accurate and aren't afraid to take out all of their friends as long as they catch you in the explosion. Zoom in like you did before and take him out as soon as possible. Finish off the rest of the Flood like you did the previous ones. Keep on them with the pistol until they get close and bunch up. Grenade them then follow up with the shotgun. This time there may not be any Sentinels left to distract them, so be ready to take a lot of heat once the last of them goes down. Back up and keep doling out shotgun shells until they're all dead. Scavenge the area thoroughly and find that rocket launcher. Switch out your pistol for it. Rocket launchers on this level aren't really worth saving. Pick them up and use them on the first enemies you see then toss them and get your pistol back. You also don't want to run all the way back here to get your pistol. So keep switching out the two weapons, moving forward. If you get the rhythm right, you can move pretty quickly, constantly tossing one weapon in front of you while picking up another. Of course you have to stop when you need to turn, but you don't need to carry it far. Just past the next big door is fine. This trick is obviously very useful throughout the game, but it's especially useful for getting rocket launchers around on this level.
So now regardless of which route you took, you should be ready to move through the giant door once the Monitor opens it for you. Again, you will have a choice: right or left. This time right is the only way to go. There's nothing but infection forms to the left. Get that rocket launcher ready (if you have one). Run ahead and as you round the corner a whole bunch of combat forms will come running up a ramp in front of you. Send a rocket right into the middle of them and watch them fly. Use up the rest of the rockets on anything nearby then go back and get your pistol. If you don't have a rocket launcher, a well-placed grenade works just as well. Make sure the area up here has been thoroughly cleansed then grab the health pack on the backside of one of the nearby pillars. Next, head down the ramp to the tunnels below.
There are several carrier Flood down here, but you can take them out from a safe distance. Just be careful going around corners. As you approach the end of the tunnel, get your shotgun ready. As you move out into the open, a combat form will drop down right in front of you. Put a shell into him then switch to your pistol and retreat into the tunnel. Zoom in and take out anything that comes down after you. If a whole lot of Flood come down the tunnel at once, you may have to back up all the way around the corner and shotgun them as they come around. Move ahead up the ramp and look for more Flood. Retreat into the tunnel as necessary and eventually the area up here will be clear of Flood.
Move around the corner ahead to trigger the next wave of Flood. They will drop down from near the ceiling. Back up to where the ramp meets the floor you're on and wait for them to come around the corner. Apply shotgun shells and grenades as necessary. Once you've dispatched each wave, move up to trigger the next. Use the same tactics for each and if things get too hot, feel free to move back into the tunnel. It's much safer in there. Scavenge through the bodies once they stop coming.
Move ahead, but be careful. The path will take a turn to the right after you pass the vents the last guys came out of. At the end of this next hall lurks another Flood with a rocket launcher. There are also many carrier forms nearby, so zoom in with your pistol and carefully peek around enough to pop a few carriers. Keep moving around sniping anything you can. If they come after you, use your motion tracker and run them into either a grenade or your shotgun. If you can take them all out from a distance, that's even better. If you see a rocket coming down the hall, run away! Once these guys are toast, move ahead and collect the goodies. There's a health pack at the end of the hall on the left. There should also be two rocket launchers lying around. Don't pick up either yet. Move around the corner ahead and a small group of Flood will come after you. These guys aren't worth wasting a rocket on. Just back up around a corner and shotgun them. One infected marine will stay on the little wall around the corner instead of pursuing you, so peek around and snipe him down. Now you can switch out your pistol for a rocket launcher. Be careful not to run over one of the launchers while you're carrying the other. You'll pick up the extra ammo, but waste any rockets that are already chambered. It's not a big deal, but, hey, one rocket can be a big help. So put your pistol and one rocket launcher in a safe place then move ahead.
Move through the section with the ramp to the section with the pillars and things. As you approach, a few carriers will appear to the left and a few combat forms will drop down from the vent on the right. Send a rocket towards the middle of the combat Flood then retreat. Get a bit of distance then finish off anybody who comes after you with your shotgun. Reload everything then do it again. Once you run out of ammo in one rocket launcher switch it out for some random weapon lying around then switch that out for the other rocket launcher. Now you'll have more rockets and can continue blowing things up. Grab your old pistol once you've expended both rocket launchers. By the time you get your pistol back, there shouldn't be any more Flood coming out of those vents.
Move to the end of the hall and a whole ton of carriers will appear out of nowhere. Turn back the way you came and retreat, carefully blasting through a couple of them on the way. Once you're quite certain all of the carriers are behind you, turn around and pick them off from a distance. As long as you aren't surrounded, these guys should be pretty easy.
Now you're clear to move onto the elevator and head to the next floor. You'll encounter some minor resistance if you wait around near the elevator, but nothing really nasty. Just run straight ahead and you'll breeze past these losers.
But I Don't Want to Ride the Elevator!
Step off the elevator and head into the hall. Moving along, you'll pick up a shiny new batch of Sentinels. Lead them ahead around the corner and you'll encounter a small group of Flood. The Sentinels can sometimes win this battle by themselves, but you'd better give them some help. Just run up and off a couple of Flood nearby then let them finish off the rest. After they've stopped firing, pick off any Flood that may be out of their sight. There's a health pack up on the little sloped wall on the right past the ramp. And there's a rocket launcher tucked away at the end of the hall on the left. Look for it next to a Flood corpse. Rocket launchers are always handy, so you might want to bring this along. Try the trick of continually dropping weapons in front of you as you move to get the launcher, pistol, and shotgun all down the ramp and into the tunnel with you. Be sure to scavenge here as well, since you can get shotgun shells and pistol ammo.
Down the tunnel and around the corner is a long corridor with several combat forms at the end. Zoom in with your pistol and take them all out from a distance. Move forward a bit and a horde of infection forms will appear. Shoot a few bullets into the mass and let the rest pop themselves on your shield. There are a few carriers around the corner, so peek around and take them out too.
As you move ahead you'll see a ramp leading up out of the tunnel. There's a battle starting at the top that you may not want to get caught in. Peek out and try to gun down a few Flood, but retreat into the tunnel whenever things get even slightly nasty. Now would be a good time to use those rockets if you lugged them up here. Shoot them all off, then run back and pick your other weapon up again. Take advantage of the Sentinels as long as you can, but once they've been destroyed, get back in the tunnel. You can take out the rest of the Flood from here. Snipe with the pistol then back up around the corner and shotgun the rest. Scavenge. You know the drill.
Head away from your last battle and down the long hall. Keep going until you get the message to hold your position until the Monitor returns. Yeah, right. Don't hold your position. Run like hell back to the long hall you just came from. A big pile of Flood will come after you so toss grenades, aim for carriers, shotgun everybody, and keep retreating as long as necessary. After you finish off this initial wave, wait around in the hallway. Don't go back to area where you triggered the last group unless you're looking for another big fight. It may take a couple of minutes, but eventually the Monitor will come back and open the door ahead. Once he does so, you can move ahead without triggering any enemies. Snag the health pack if you need it.
Once again you have a choice of paths. You can run up to the edge of the chamber turn left and jump over the little wall. This maneuver was a good shortcut on the previous floor, but this time it will put you in a bad position, so I'm going to suggest you proceed the normal way.
Around the corner ahead to the left is yet another group of Flood. One of these guys has a rocket launcher, so gun them down fast. You might not want to use a grenade since that will blast the rocket launcher far away, and you're going to want it soon. So put these guys down quickly with your shotgun then toss your pistol and pick up the rocket launcher.
Ahead another small group of Flood will come running towards you. Use those rockets. Just blow these guys away. Go back and grab your pistol when you run out of rockets.
Move ahead. A few Flood will jump up onto some blocks in the area ahead. Send your last rockets at them if you have any left, then switch to your pistol. Take the rest of these guys out like normal. Pistol at long range then wrap them around a corner and hit 'em with the shottie.
Head through the big door and grab the health pack and grenades if you need them. Ahead a few Flood will jump out of one of the little trenches at you. Put each one down as they land then back up around a corner. More Flood will come around after you, but a few shotgun shells and possibly a grenade should finish them off easily enough. Peek out and snipe down the one Flood on the right wall that didn't hop down with his pals.
Ahead the path will turn left and multiple waves of bad guys will fall down from vents near the ceiling. You've been in situations like this before. Retreat so you only face one wave at a time then deal with them as usual.
After you've cleared these guys out, you have a bit of a walk. Keep moving along and you'll come upon some Flood bodies. Scavenge around a bit. There should be some ammo for what you're carrying around here.
After that, you'll come to a big door that's open just enough for you to get through into the next room. As you walk through the door, look to your right. There's a little alcove there that you will be retreating back to very shortly. In this room there are three health packs. Two are behind the columns to the left. Grab one now if you want. Once you're set, walk under the Monitor. He's waiting by the other big door. As soon as he moves away and you get the message to wait, run like hell back to that little alcove. You'll see the door you came through closing. Tons of Flood are about to fill this room. And if you kill them, more and more waves will come. This battle can be one of the toughest in the game. Or it can ridiculously easy. If you stay in that alcove and keep your mouth shut, they won't ever know you're there.
Wait around for a couple of minutes and eventually the Monitor will come back and open the door ahead. The Flood will all follow him through the door and begin battling a contingent of Sentinels. While they're running away, you can move in behind them and nail them once they bunch up under the Sentinels. Toss a grenade into the middle of them and start blasting with the shotgun. If you keep getting your butt kicked here, just wait a little longer before coming out of your hiding place and the Sentinels will do more of the work for you.
After everybody's dead, scavenge and look for a rocket launcher. There should be one among the bodies. Snag it and drop your pistol, but of course remember where since you'll be grabbing it again in a second. And don't forget about those health packs back in the room of death. Go grab one if you need it.
Rounding the corner the Sentinels will begin firing upon yet another group of Flood. Wait around for a second and the Flood will tend to bunch up on the ground. Send a rocket right into the middle of them. Wait around a few more seconds and the Flood will just keep coming. Once they've bunched up a second time send 'em another rocket. Fire off any extra rockets then run in and take out any survivors with your shotgun up close. Go back and grab your pistol again.
Keep moving and you can run right onto the elevator to finish out this floor. A few pathetic carriers might drop down nearby, but I think you can handle them.
Fourth Floor: Tools, Guns, Keys to Super Weapons
OK, last floor. Move ahead into the hall. There are several carriers hiding around the corners here, so be cautious. A Sentinel here will provide a bit of help, but you'll have to pop most of these guys. There will also be a health pack right in front of you as you step into the hall. There will probably be a carrier left at the end of the hall as well, so check those nooks and crannies before moving on.
This next battle is a tough one, but use the same old tactics and you'll come out on top. Run around the corner and a big wave of Flood will drop down from vents near the ceiling. Sound familiar? Just fall back like normal and take them out as they come around the corner after you. These waves are pretty big and there are a lot of them, so be generous with the firepower. Retreat all the way back to the elevator if you have to. After a great many waves, they'll stop coming and the way will be clear. Scavenge around thoroughly then move ahead.
Head down the tunnel and take out any carriers lurking down here. This next battle can be very annoying because it's a trap. As you exit the tunnel, several carriers will spawn behind you. So when you try to retreat, BOOM, you get blown to bits. You can handle the situation one of two ways. You can just not retreat into the tunnel. Run up and dive into the melee, blasting a few guys, then running around a corner up there for safety. It's actually not as difficult as it sounds. This is the easier route in my opinion. Many, many Flood will come after you, but you won't face more then a few at a time and as long as you stay around a corner, you'll be fine. If you're more methodical and need the shelter of the tunnel, you'll have to run up and peek out then run back and blast the carriers waiting in the tunnel. Once they've detonated, run through the mess and get you pistol ready for any pursuers. A few Flood will probably come down with you and you can take them out. Then you'll have to repeat the process again, and again. There are many waves of carriers set to spawn behind you, but eventually you can kill them all and clear the path behind you. Each time you run back through them a few Flood will follow. So you should be able to take out the rest of the guys up top with little trouble. Once you're ready to leave the tunnel for good, there's a health pack lying around for you. And there's another one in the ditch ahead if you've already used that one up.
Move along around another corner or two and eventually a slew of carriers will drop down all around you. Turn around and blast your way through a couple to clear a path then take the rest out long range once they're all safely on one side of you. Past the carriers you'll run into another small group of Flood as you round a corner. Take these guys out as usual. Snipe a bit if you can then hit them as they come around the corner after you. There are several carriers amongst the group to work with as well.
Look behind the pillars ahead and you'll find a rocket launcher and a health pack. Swap out your pistol for the rocket launcher and toss them around until your pistol is in a more open area closer to the big door ahead. You're going to want it back after you fire off those rockets.
Alright. This is it. The last battle of the level. Move towards the big door to begin. The door will only open part way before Flood start jumping through to get to you. Put a shotgun shell into each of these guys as soon as they land. Once the door fully opens, a deluge of Flood will come after you. Toss a couple of grenades and shoot two rockets into the approaching mass. Retreat and reload. Don't get backed up into a corner. As long as you stay on the path you used to get here, you'll be ok. Just keep backing up and going through rockets. Once you're out of rockets, run back and grab your pistol. Keep retreating and reload after each couple of shotgun shells so there isn't too long a pause in the damage you're dishing out. Use up all the grenades you have if you need to. Toss them at any groups. Eventually, you'll be left with a long string of Flood corpses and little ammo. Scavenge through the bodies moving back up to the last big door. Just after the door there are a couple of Flood perched up on ledges on either side of the door. Toss a grenade at them or gun them down with your pistol. Be careful not to move into the next room or you'll risk triggering the next wave.
Now once again you have two choices. An easy route and a route that allows you the satisfaction of killing everything. I'll go over the easy route first.
If you want to just run for it, head out the door and take a left. That's right. Not towards the index. Just run as fast as you can into the chamber on the left. Once you're in there, head towards the edge overlooking the central area with the index. Once you're on the edge, turn ninety degrees to your right and you will see a little sloped wall. Run up and jump on top of it. It's high, but you can get up there. Fall down on the other side and make a beak for the index. You should get there fairly easily.
If you want to be more thorough, you can deal with the rest of these Flood in the traditional way. Move into the chamber and more waves of Flood will come from the area near the index. Fortunately none of these are as nasty as that initial wave. Just keep cool and go through the ammo. Scavenge whenever there is a brief lull. There are many, many waves of enemies, but none of them is really that difficult. Just stick to your old tactics of retreating and wrapping them around corners and you'll do fine. Eventually you will have killed every Flood in the area. Once you've accomplished that, the index is yours for the taking. Saunter on up and snatch it.
Congratulations. Now that you're a both a seasoned Flood-killer and Covie-killer, the rest of the game should be a breeze, right?