Act 1: C3 - Rolling Thunder

As it starts, turn around and a paper is on the wall. After you watch a lot of friendlies go down, it will soon be time to defend your rig; it's after you collide with a rock. I know there is a turret you can man, but just stay put and use the Lancer to shoot down the mortars as they fly in. So you had best stand back and try to hit them when they are near. There are about 8 or so that all come from the right side, the direction of the trail.

After that, get on the turret and look to the left to shoot down a Reaver, then shoot at any others you see. Then it should be smooth for a bit.

Once you are off, quickly run left for a paper and some ammo, and be sure to pick up those frags. Take cover and then start taking out the enemies that have come out of emergence holes. To close these you need to put a frag in them, but for now there are too many enemies. You could toss both to the left and right of the middle of the group and hope you get them closed, but the enemies seem to stop coming after a few come out. Then you need to take out the enemies to the right of you and up high. Soon after more emerge from the ground where the first came, but you'll be able to run to the rig shortly, so do that.