We'll be using this hideout to lure the remaining Combat forms to us. Just take them one at a time and it shouldn't be a challenge for you at all. Now start moving up the slope. Dealing with the Ranged Flood becomes next priority. If necessary, you can cloak yourself while you try and stick them. This serves as the most effective method. Be sure to reserve at least one of the cloaking devices for later. Take the following route to the upper level.
Stay in the caves as much as possible to avoid the Ranged Flood stationed higher up. Kill the Combat Forms in the final cave up top. Then clear the area of all immediate threats. Either knock the remaining Ranged Flood off the walls or kill them with any sticky-based grenades you have left. See, that wasn't so bad. You just needed to take your time. Head through the next door into an empty room. Ready your energy sword and enter yet another Flood door and into the Reactor Room. Your approach here is the riskiest portion of your time here. Proceed as follows.
Kill the Brute Combat Flood and Stalker that stand in your way. Ahead will be a few unshielded Elite forms, two Ranged Flood, and a Tank. Wait until you're right up close to the Ranged Flood before using your camo. Then deal with the other Ranged Flood, and lastly the Tank. Now your attention should be focused on the Elite Flood. Use your sword to deal with them. There should also be a few Stalkers standing idly around. Kill them too. We want to clear this entire room. Grab the Deployable Cover near the door and head towards the center of the room. A few Pure forms will be standing guard. Only the Tank forms will fight back, but kill them all anyway. Spare nothing. Drop the Deployable Cover on the catwalk between you and the exit in such a way that it blocks at least some of the firepower unleashed by future Ranged Flood.
Nor Hell a Fury...
Exit the room and back the way we came to avoid further abuse from Gravemind. He sounds pretty pissed, so I say we blow this joint and make a grand escape.
Just take them as they come. Your main concern will be any Combat forms using maulers or swords. Take them out from a distance if you can. Use the carbine against the Brutes and plasma grenades against the Elites.
Active Pure forms are top priority. Rush them with the sword. Then smash through the Combat forms one at a time. There should be at least two Elites and possibly a Brute if you didn't already headshot it from a distance. You'll also notice a ton of Stalkers just standing around doing absolutely nothing. Do you like points? If you do, kill them. No sense in taking any chances this far into it anyway. Now backtrack to the Cortana side of the room. Temporarily switch out your sword for the idle Brute shot. Head back to the center and activate the reactor via the bullet-shaped control panel. Three fragile columns will expose themselves. One explosive from the Brute shot per column will do the trick from right where you're standing.
Gravemind will scream in rage, and then the whole place will start to explode. From what I've been able to observe, the explosions themselves don't seem to hurt you curiously. Your exit door will shortly open, revealing a mauler-wielding Brute form. From where you are he is of no threat. Headshot him with the carbine to keep it that way. Backtrack to the Cortana side, retrieve a fresh sword from the crate, and grab the last cloaking device.
I've found that because some of them tend to be using Brute shots and maulers, it's best not to try and engage them. Run through the Carrier room and carefully sneak into the Projector Room. If you're quick enough, you should notice a few Combat forms retreating back into the opposite doorway. Wait until they're out of range, then sneak underground (where we were the last time we fought in this room) and trade out your sword for the full plasma pistol. Head back up top and headshot as many as you can from a distance. They'll all be in that doorway area huddled together. You might even be lucky enough to combo shoot the Elite forms. When a certain number are dead, they'll all start charging out at once. Retreat back to the opening from which you entered and wait for them. This is why we set up those Deployable Covers, to narrow their options.
Plasma punch the Elite forms, and crumble the Brutes. If you can headshot any unshielded forms before they get there without putting yourself in harm's way, do it. Once they're dead, we'll need to trigger the second wave. Head back towards the opposite end, and the rear door will open. Many times this group will include an Elite form armed with a sword and a Brute form wielding either a Brute shot or mauler.
Kill the latter from a distance with the carbine, and combo-kill the Sword Elite. If you become too overwhelmed, back up into your protected passageway and take them on one at a time. Only one last thing stands in your way here. Momentarily grab one of the swords and proceed towards the door again. A pane on the right side will explode open and a hulking Tank will come lunging from the opening. By now you should be nothing short of a Tank slayer, so dish out the hurt with a little slice n' dice action. Before you leave, abandon the sword and grab a plasma pistol and carbine. You should already have at least one of these. If you're out of plasma ammo, them grab the sword instead, but plasma punching is far more effective against the Elite Flood.
From here on out we'll be backtracking to Pelican Hill. Don't worry. It won't be so bad this time around. After dropping down, head towards the spiral Flood door further to the left. Open it, but don't go through it. Hang back and gun down the shieldless Flood from afar. If you have stickies left, now would be a good time to use them against the Carriers waddling around in the same corridor. Once all threats have been neutralized, continue your Pelican hill pursuit and trigger the last Flood door you'll encounter.