Once you out of the tunnel (that is not a natural formation) and headed toward the closest lifeboat, there's an alternative route to extend your life. This is really only necessary on legendary, and will work with the walkthrough.
First you need the S2 AM that a marine left behind. Go in the entrance that normally has 3 jackals and ride around the trees on your right. The point is to not let the Covenant shoot at you by speeding past them. Once there are no more trees you'll see a cliff opposing the structure. We're going to climb it. There's a visible 'Y' faded into the grass, ride up the left side of it. It's a short way up and you'll find it very pleasant on the top. Now, we have a sniper rifle, a pistol, and a warthog. The way I see it, you have three options.
1) You could back up the warthog to the edge and use the turret on them. This will not work on elites, most of the time. And jackles will repeatedly loose charged shots at you. It may also become frustrating if you're enemies take cover too early, as it isn't convenient to reposition the warthog.
2) Use only the S2 AM. You do have enough ammo, and it is certainly the easiest solution. But if you're a true sniper you'll feel stupid using those expensive rounds on grunts. If you want to conserve ammo, a mixture of this and #3 is usually my choice.
3) Pistol. Considering the large plateau you're on, moving to get closer shots is very easy. The only problem that may arise is if elites repeatedly dive under things. This does not get frustrating however, as you can quickly reposition yourself for a nice clear view.
And there are some cracks that you should be aware of:
Sometimes jackals will go underground, thus making it impossible for you to kill everyone and free the marines through a checkpoint.
Once the elites are dead, no more will appear. So you may find it more appealing to slide down to kill the rest close-range.
If you accidentally fall down on you own, and finish anyway, foe hammer will bring you a second warthog. If you bring your warthog down and park it outside, she'll do the same. Thus feel free to use the walkthrough's idea.
If you come up after the marines arise and are taken away, banshees attack.
It's very helpful to shoot from beside the warthog, as when you don't know if you scored a kill or not, the marine will comment on a good shot.