(1) Open the entrance door to begin the level. Work your way around to the 2 opposite-facing doors, killing the 3 facehuggers along the way and shooting the 2 single crates for shotgun and pistol ammo. Lead these 'huggers back to the entrance and kill them there if you don't want to deal with them again later in the game, even though that means you have to kill 2 of them more than once. At the doors, you must activate the switch on your left before you can open the left door.
(2) A facehugger will attack as soon as you enter--lead it down the stairs to the end of that hall and kill it, then go back into the room, grab the auto-mapper, and activate the switch inside (while leaving the derm patch for the moment). Now you can open the door across the hall.
(3) Grab the shotgun, but don't blow the crate yet or you will have to kill the facehugger inside again later on. You will now want to stick with the shotgun for the rest of the level, except where noted.
(4) Go back toward the stairs and approach the row of barrels on the left. A 'hugger will come out of the wall on the other side; shoot a gap in the crates to aim through and kill it with the pistol from the safety of the barrier. Now blow the barrels. Go down the now-cleared hallway and stay to your left until you have almost completed a circle (save the derm patch and medical kit for later).
(5) Blow the barrels and crates in front of you--this will release 1 facehugger, and the last one you killed will be revived. After killing them, heal any injuries by grabbing the med kit and derm patch you passed in the hall and go back to the 2 doors. Get the derm patch in the left room. Now blow the crate in the right room (with pistol) and kill the facehugger.
(6) Approach the barricade between the 2 rooms. The second crate from the left houses a facehugger--release the critter with the pistol and try to kill it to the side (by shooting over the barrels with the pistol) so it is not in front of the door to the room ahead. Fire a few rounds from your pistol (for Heaven's sake--conserve that shotgun ammo!) through the gap in the middle of the barricade to draw a nearby warrior to you on the other side of the crates and barrels. You can use the shotgun and fire at it from relative safety (it rarely hurts you across the barricade) when it comes close, or blow the barrels at the right moment to kill it. However, it is better (but more dangerous) to blow the barrels and let the warrior come after you so you can kill him away from the unopened doorway (what you are trying to do is leave an acid-free line of retreat from the door).
(7) Now open that next door and immediately back up. Lead the warrior that comes after you back away from the door and kill it. Go back to the door and activate the second warrior and kill it.
(8) Now enter the room (be aware that once the door closes behind you, you cannot reopen it). There are 4 facehuggers running around; draw them out of hiding one at a time to kill them. Now blow each crate from a distance (2 will contain facehuggers) and grab the powerups. Go through a gap in the crate "wall," get the pickups (including the battery you will need shortly), and kill the facehugger that comes from your right as you exit the room through the far door. Walk to your right and get the shotgun shell and clip. Now go back the opposite direction and get the 9mm and the shotgun shell. There will be 2 wall recesses on your right as you go: the first contains 2 'huggers (make sure you draw 'em both out) and the second (past the shotgun shell) contains 1.
(9) Now go back and enter the unexplored hallway on your left. You will find 2 facehuggers in wall recesses and 3 more (plus 1 warrior) waiting for you out in the open.
(10) After you kill them, activate the battery-operated panel at the end of the hall and open the door to the exit. Round one!