A fairly tough level, but still not really worthy of the final Alien bonus. Still....

From the start get onto the ceiling. Climb up to the walkway and follow it around to the lift (not the stairs). Take the lift down and kill the marine there, then run VERY QUICKLY across the walkway which goes right. Kill the marine guard there and hit the switch to deactivate the turret. Run back across the wall to where you just came from. Climb up the wall or take the lift up. Follow the walkway around and either climb the stairs or the wall. Go down the next flight and jump down into the mould. Go through the door. Take the first tunnel on your left and hit the switch to deactivate the turret. Go town the tunnel and take the first left. Go through to the end and turn left. Wait for the elevator to come done and ride it back to the top, or climb the shaft once it lowers. There is a Predator in this room, so kill it. Don't worry about the switches because the turrets can be avoided. Jump down to the bottom and go through the hole in the floor. One door here has a red light above it. Take the opposite door of the room. The tunnel on the left has a series of barrels in it. Destroy one and back off to watch the chain reaction. That will shut down the fan. Run straight ahead and through the opening to the Predator's starting point