(1) Open the entrance door, face left, and sidestep into the hallway to meet the warrior that charges you. Kill him and then go up the stairs on the right until you see another warrior.
(2) As soon as you spot him, turn and run back to the entrance. Re-kill the first alien (quickly!), then turn and kill the one hot on your trail. You will see these 2 yet again at the end of the level. This doesn't just seem like a lot of trouble--it is! However, this will save you from dealing with that second alien repeatedly (at the same time as a whole herd of facehuggers) later on.
(3) Now go left up the stairs. Down this hallway you will find 2 facehuggers and a warrior. You will also have to make timed dashes past the steam vents to avoid burns. Don't miss the auto-map and multiple crates here. Follow the hallway, staying to the right until you reach a barricade at the top of some stairs.
(4) Blow the obstruction, go up the stairs, and activate the panel ahead and to your right.
(5) That will allow you to open the door now directly behind you. When you do, a 'hugger is waiting. There will be 6 more down past the long stairway. You will have to make several trips up and down the stairs to activate them all (and earn the elusive 100% for all aliens killed), and you will have to kill most more than once (it will be easiest in the long run to lead them all to the top of the stairs and turn right down the hall to kill them all there; this also happens to be where that second warrior you killed would have kept repeatedly reviving).
(6) This done, open the door past the bottom of the stairs and immediately begin firing as you back up around the corner and up the stairs. Four warriors are after you! Having dispatched them, enter the room, blow the barrels and crates, and throw the switches (which activate more doors). One crate holds a 'hugger. Make sure you grab all goods, including the battery and seismic charges.
(7) Now go back up the stairs (there may be some revived 'huggers waiting for you) and blow the barricade on your immediate right, releasing some pickups and 1 facehugger.
(8) You can now pass down to a room containing 2 facehuggers. Kill them and blow the crates, which contain some goodies and another pair of 'huggers. Before exiting, grab the med kit and derm patches off the shelves opposite the door.
(9) Leave the room and blow up the crates on your left, which will release a single facehugger. Go back up the stairs you came down (there may be some revived 'huggers waiting for you--yet again!), go left, and go down the stairs past the switch. Take the first right at the bottom of those stairs and open the right door.
(10) A warrior is waiting for you--don't disappoint him. Grab the clips and patches off the shelves, activate the switch, and open the doorway on the opposite side of the hall.
(11) Kill the facehugger and 2 warriors inside, flip the battery-operated switch (it opens the exit), and take the med kit and derm patches off the shelves and table. Exit and turn left; a warrior will be pacing back and forth behind a crate-and-barrel barricade.
(12) Blow 1 crate to expose a barrel, and then lure the warrior within range and blow the barrel, killing him. Back up immediately--in addition to the barrel blast, 2 more facehuggers have just been released! Continue through this new opening and turn left to the very end of the hall.
(13) You are facing your first secret wall. Blow it with a seismic charge and enter. You will find the flame thrower, hypo pack, and 2 shotgun shells.
(14) When you open the door at the end, there is a warrior there. I prefer to kill the warrior first and then pick up the hypo pack; this gives me a few more seconds of invulnerability to make it to the exit unscathed. Having grabbed the hypo pack, run out the door and stay right all the way back to the exit.
(15) At the exit there will be a new warrior waiting for you as well as the first 2 you killed on this level. Since you don't know which is which, kill them all if you hope to see a 100% kill on the MISSION ASSESSMENT screen, and then exit.