The briefing "Go out there and kill things." pretty much sums up this level. In fact, you end up killing a lot of things. This is a worthy end to the Marine's campaign, and in my mind is without a doubt the hardest level in the entire game. Enjoy.

You start off in the airlock of the Area 52 installation. Walk forwards enough to make the door ahead open. You see a marine guarding the entrance as two aliens pounce on him. Save him a painful death and shoot a grenade at him just as the two aliens jump. It should wipe them both out. Move out onto the platform, then press the button to your left. Step onto the lift and ride it down. Walk down the steps and enter the tunnel on the left at the end. Make your way up the stairs and through the door at the top. Kill the two aliens here and then collect the two Pulse Rifles against the right-hand wall. Make your way up the stairs and enter the area with the Lotor. Go to the far end of the area and enter the tunnel on the right. Take the first right up the stairs and into the control room. Collect the Minigun in the far right hand corner. Go back down the stairs and then head right, through the door. There's a room on the right where you witness and marine get slaughtered by two aliens - kill them both and then p the switch in there. I'd recommend you save it now. Proceed down the corridor and through the door at the end. Instead of going straight up he steps ahead, go underneath them - you'll see three aliens, all neatly curled up and ready to spring to life. Line your aim up, then start shooting - with any luck you should be able to kill them all straight away. However, a Praetorian then appears, so pull out your Minigun and waste him. Now head up the steps and along the walkway before entering the tunnel at the end. Head round and down the stairs and you'll emerge in the next checkpoint. Kill the aliens here then head for the steps in the opposite corner of the room to the one you entered. There's also some Armor next to the gate. Proceed up the steps and across the two walkways, then along the pathway on the other side and pick up the Flamethrower at the end. Head down the steps and through the door, then kill the two facehuggers in here (try to attract them into the doorway then flame them with the Flamethrow).ress the button next to the door, then an alien and two Praetorians will run up the steps to greet you. Dispose of them however you please. Now head down the steps and wait. There is a sentry gun that will attempt to take out the attacking force, which consists of a few aliens and a Predalien. However, normally some get past, so be careful. Proceed past the sentry gun and up the stairs. When you near the top an alien will try to attack you, but the sentry gun should take it out. Continue up the stairs, up some more stairs, then along the pathway. Here there is a lone marine who will get attacked by another "band" of aliens - a Predalien and a couple of aliens. My suggestion is to run back to the sentry gun and let it take them out. Anyway, keep progressing along until you reach a door. Open it, then kill the two aliens that are on the other side. Now switch to the most powerful weapon you have, which will hopefully be the Minigun. I strongly recommend you save it now. Jump