Pull the lever on the right to open the door. Go up the steps on the far left hand side and pull down on the two switches there to open the hangar doors (the ones at the bottom of the stairs). Go through said doors, then head past the cryo-sleep beds, past the lockers, then round the corner and past yet more cryo-sleep beds. Now, walk forward until you hear a facehugger scrabble out of the lockers, then blast it with a grenade from your Pulse Rifle. Make your way round the corner and then pull the lever on the far left to open the door. Head through it, then go round the corner, past even more cryo-sleep beds, then head left. Pop into the showers to collect a Smartgun and occasionally some armour. Now exit the room and head over to the far left corner. There's a vent grate - shoot it and enter the vent. Shoot the two aliens that appear, then continue down the vent, shooting the grate at the other end. You emerge in the mess hall. On the far left are some bonuses, collect them then proceed through the doors it opposite corner. Head down and round the corner, then pull the lever on the right to activate the lift. Ride it up, then go forwards and enter the passage on the right. Keep on following it round until you reach a dead end - here there is a lever. Pull it to activate the cooling systems. Head back down the passage and then proceed through the doors on your right. You're now in the reactor. Continue through it, then head through the doors at the bottom. Enter the next area and blow up the barrels on the far left. Head through the gap were they were, then walk up the stairs on the very far right. Pull the lever on the left, then enter the doors on the right. In here, it may be tempting to use the Smartgun, but I'd recommend you actually use your Pulse Rifle. Make your way through the emergency exit, killing the Praetorian at the end if you're doing it on the harder levels. At the end go through the doors and then ride the lift down behind the barrels. If there's one there, collect the Sadar on the left at the bottom. Head through the next set of doors to enter the hangar. Here there's a Predator, but next to those missiles on the far right (next to the hangar door) there's a minigun. Collect it, then waste the Predator. Now quickly make your way to the top of the steps on the opposite side of the room (with a door at the top). Wait for the Predalien, then dispose of it with the minigun (you should still have loads of ammo left for it) or the Sadar if it was there. Make sure you get your sorry ass to the top of those steps for when the doors open. If it's available, I'd suggest you switch to the Smartgun right now. Make your way through the changing rooms, past all the lockers, benches etc. At the end there's a Praetorian (always). Waste it with the most powerful weapon you have. Then pull the switch on the left and ride the lift up. Make your way into the next area - but first you'll need to activate the lift. Make your way round, killing any Praetorians (yes, that's plural) you meet until you reach the switch. Press in switch, then make your way back to the lift and ride it down. Step off it to finish the level. Phew!