(1) As soon as you walk out the entrance door a warrior will attack from your right. If you can, take him out with a seismic charge, or he will be back later. This is most easily done by running straight out the door and down the main corridor to put some distance between yourself and the enemy, then pivoting around for a clear, straight launch.
(2) Proceed up the corridor the warrior came out of. Past the first corner 2 facehuggers are waiting within the wall. Toast them with the flamer and continue around the next corner. A facehugger will emerge just before the first pair of lockers. Kill it and go past the second pair of lockers, where another 'hugger lurks in the right wall. Now go back to that last pair of lockers: the left-hander houses your first chestburster. Blow the locker and kill it with either the shotgun or flame thrower. Just ahead now lies a warrior. The middle locker in the trio you will find on your left (just before another corridor) holds a shotgun shell and ID card. Grab them and continue ahead, bypassing the left-hand corridor. A facehugger is in a wall recess on your left. Kill it and move forward to the pair of lockers dead ahead. The left hand one holds a shotgun shell and clip. Around the next corner another 'hugger is hiding in the corner behind the lockers on your right. Another warrior will attack when you continue on.
(3) Kill him and enter the doorway on your left. While either entering or exiting this room, 1 or 2 facehuggers from around the next corner may come after you, so be prepared.
(4) Inside are 4 cryotubes; going clockwise from the one closest to you as you enter the door, they contain nothing, a chestburster, a med kit and ID tag, and another 'burster. It is best to back up after opening a tube with a 'burster inside since these aliens move very rapidly and are hard to hit at close range. Now exit the room.
(5) Five facehuggers are in the locker-studded corridor ahead; 4 are in the walls, and 1 or 2 of them may have already attacked you near the cryotube doorway.
(6) You are now exiting this smaller corridor into the straight, primary corridor of this level. Two security guards are protecting the unopened exit door; kill them--they were infected anyway. (If you didn't encounter all 5 'huggers in that last corridor, the 2 [usually] stragglers will emerge when you start shooting at the guards, but you may have to backtrack for them).
(7) Now go down the primary corridor. Enter the doorway into the darkened room on your left. See the battery? See the shotgun shell? There is a warrior around both corners. Kill the pair and grab the goods.
(8) Exit, turn left, and enter the doorway on your right. Kill the 2 security guards, positioning yourself so their stray shots don't break any of the glass walls and release all the aliens inside prematurely. Now throw the 4 switches, which activate the doors into the 2 glass-walled laboratories, open the exit door, and power up the room's lights.
(9) Now go to the smaller glass lab and open the door. Five facehuggers will emerge; flame them, and don't miss, or you'll get overrun. (You could also shoot the glass walls to get at them). The auto-mapper and an ID tag are on the table inside.
(10) Now enter the door to the other glass-walled room. [BUG! In this room I once discovered an invisible gap in the glass wall facing the smaller glass room. I was able to walk in and out of the room through it, and the warrior inside came out through the gap to get me! I assume this is a glitch that rarely loads.] Three facehuggers and a warrior are running loose; if you can kill them all inside the room (and without hitting any of the glass walls), they will be contained and not present any future problems. Unique to this level are the incubators that you can blow open. The clue? One is already blown open and has a derm patch lying in front of it. Blow open all the other panels; each contains a facehugger except for one that is empty and another that yields a second patch. The trick is to avoid the released facehuggers in the tight quarters of the small room. Now throw the switch to power up the door so that you can get back into the main room. Before leaving, blow the second and fourth lockers on the far wall for some shotgun shells and an ID tag. Exit and turn right. You will be heading back to your starting point on this level. If you didn't kill that first warrior with a seismic, you'll have to kill him again now.
(11) After doing so, open the door to the room on your left, and be ready to nail the chestburster on the other side of the door. Kill it and proceed into the final room. The words "Lift activated" will appear--you have just opened a secret area by throwing all the switches and then entering the last unexplored area of this level. Be patient--you still have a little more work to do here first! In the adjacent room through the open doorway is another loose chestburster. Kill it and deal with a final 'burster ensconced in the first of a row of 4 lockers (an ID tag comes out of the same locker). In locker 3 are a med kit and clip. Two more med kits are lying on tables in the room, and a fourth kit is in the left-hand locker of the pair on the far wall. As you open the door to exit this room, the first warrior of the level may be revived yet again if you didn't "charge" him earlier.
(12) Now move left out the door; on your right will be a ramp leading down to the secret room that you activated earlier. Kill the 2 facehuggers down the ramp and grab the goods: an ID tag, shotgun, pulse rifle, acid vest, and flamer fuel. With the pulse rifle, you are really ready to rock! Switch to it, go up the ramp, and head left all the way back to the exit.
(13) Use the long-range reach of the pulse rifle to kill the 2 revived exit door guards before leaving.