(1) Alas, you will not exit this level with 100% for SECRETS. BACKED BY A THOROUGH FAMILIARITY WITH ALL OTHER SECRET AREAS IN THIS GAME, AND AFTER HOURS AND HOURS SPENT ATTEMPTING TO ACCESS ALL SECRETS ON THIS LEVEL, I HAVE CONCLUDED THAT AN APPARENT BUG IN THE GAME PREVENTS YOU FROM ENTERING THE LAST SECRET AREA HERE. Quite a letdown after coming so far, particularly if you have been doing well at the Mission Assessment screens. Try not to look too disappointed as you enter this level and move forward.
(2) You are in a short corridor that terminates in a lift that drops as soon as you step on it. As this lift descends, you will see a warrior attacking from your left. There are grenades just around the next corner, so use 1 if you are near capacity. Otherwise, the smartgun is a good choice as there are 2 magazines to be had not too far into the level. There is a single med kit in the room you drop down to if you need it.
(3) Now continue ahead until you see a grenade around the next corner. Do not head straight for it or you will never reach it! Instead, sidestep left until you are up against the left corridor wall and advance to the grenade while hugging this wall. Nab the powerup and turn so that you are now facing that left wall. Now sidestep onto the lift (there is a large, tan circle on the wall directly in front of you here). To activate it, dart forward. AS SOON AS THE LIFT STARTS DOWN, run off it--over 4 angry warriors' heads!--and immediately turn 90 degrees left to run down a corridor (you will find a pulse rifle magazine en route). By doing so soon enough, you will go OVER (rather than through) the aliens awaiting you directly below. Once you near the end of the corridor (there is a short ramp here, but do not go down it yet), spin and let them have it with the smartgun. When you have whittled this population down to only 1 or 2 warriors you can switch to a lesser weapon and conserve that smartgun ammunition. Now go after any remaining warriors near the lift area (there were 4 total). Grab the night vision goggles out of a very narrow recess that you may have to sidestep back and forth to get into.
(4) This accomplished, move back to the short slide at the end of the corridor. Walk down it partway to arouse the 3 nearby warriors. As soon as a warrior is "stuck" at the bottom of the slide, back away from the top edge of the slide slightly and dispatch it with the pulse rifle. Repeat until all 3 are dead. You may actually have to run all the way down the slide and back up to rouse the third warrior. Rarely, 1 of the aliens below will actually hop up onto the slide and come after you! Once down the slide, deal first with 2 additional warriors off to your right. Having dispatched them by the method of your choice, locate the pulse rifle magazine, 2 shotshells, and med kit in this area.
(5) Now continue ahead to another slide. You will land in a new room facing a very short corridor directly ahead. Enter this corridor cautiously in an attempt to activate the 2 company synthetics on either side 1 at a time. There are 2 smartgun magazines in this room, so you can utilize this weapon for convenience if it is near full capacity on ammunition. Each synthetic coughs up a seismic charge as he expires. Enemies eliminated, grab the goods in this area: the 2 smartgun magazines, 2 more seismic charges, 3 flame thrower canisters, a med kit, and 2 derm patches (both in very narrow recesses that you may have to "wiggle" into).
(6) There is a lift within the hallway here. It is behind a walk-through wall on your right about a quarter of the way down this corridor (the wall has 4 vertical, gray "stripes" on it). Enter and activate the alien switch. You will go up quite a distance (this is the longest lift in the entire game) and stop at a room filled with 6 derm patches and 2 shotshells. Grab the goods and move carefully out and into the corridor leading out of this room.
(7) See the grenade? See the 9mm pistol? Well, you can't touch 'em! Halfway down this corridor, the floor drops out to deposit you on a long slide. You are now facing the first tough part of this level. Off this slide are 2 secret rooms (1 to the right and 1 to the left), each with a warrior within. Move partway down the slide to activate them, then run all the way down and stay right. You will have to dodge 2 or 3 warriors to end up in a long hallway holding 3 derm patches. As soon as you near the end, STOP (there is another drop-out-from-under-you floor here) and turn with your smartgun to face 8 warriors (assuming they all followed you). Kill them all, ignoring for now the synthetic that may be shooting at you from behind. You can save ammo by switching to the pulse rifle or shotgun once there are only 1 or 2 warriors left. Leave the company synthetic for now.
(8) Time to backtrack to the slide for 3 secret rooms; you may have to kill 1 or 2 residual warriors along the way. The first secret to explore will be at the end of the recess in the immediate right corner (as you face the bottom of the slide). Lob a seismic charge here to expose 2 company synthetics, 1 of which drops a seismic charge as he goes down. Within you will also find another seismic and 2 pulse rifle magazines.
(9) Shuffling your way back up the slide for the 2 secret rooms mentioned earlier, you will find an adrenaline burst and seismic charge on the right, and another seismic charge plus 5 patches and body armor (Yay!) on the left.
(10) Now reverse your route and head back down that corridor to the drop-out floor (but don't miss the 2 med kits in the main room here if you need them). Up to 2 or 3 warriors may have revived here, depending on where you dropped them with the smartgun earlier. Dispatch them and do the same to the synth you left here earlier (he yields a seismic charge when he dies, and if you are lucky it drops close enough to the edge of the drop-out floor for you to grab it). You will lose 5 health points here (nothing to be done about it, unfortunately) when you drop down through this floor. You land in a room with a large central pillar and that is loaded with alien canisters. There are also 3 company synthetics here. Use grenades on them to prevent their revival later on when you return to this area (and don't miss the seismic charge one drops). Now blow all the alien canisters for a total of 7 derm patches. It appears that 2 canisters--the pair on your immediate left when you first drop in--block the unopenable secret area. If you shoot them and then walk up to the wall directly behind them, 2 warriors appear on your motion tracker. You can also see this L-shaped room on the map when you pause the game, but you cannot enter. Throw your controller down, have a fit, and move forward.
(11) The short corridor ahead ends in an acid curtain. Behind it are 4 loose warriors and the remaining rooms of the level. Shoot into it to activate the first pair of warriors and grenade each as it emerges from the curtain--you should have a clean shot as they move down the narrow hall. Now pass through the acid curtain (5 armor points--OUCH!) and face left. Two more warriors will attack; grenade them as well. You now face the toughest part of this level and one of the toughest areas in the entire game. Within this round room are 7 switches. Each opens the door facing it. There are enemies behind every door. There are 2 approaches, depending on the difficulty setting you choose to play:

On EASY or MEDIUM, you should open the doors 1 at a time. You do not have a choice as to which order you open them in. Activate the LIGHTED switch, turn around to explore the now-open room immediately behind you, and then (after killing the enemies and grabbing the goods), activate the next switch that lights up.

HERE IS THE SEQUENCE FOR THE EASY AND MEDIUM DIFFICULTY LEVELS (activate the night vision goggles you found earlier now).

1) Initially, only 1 switch (RED) is lit up. The room behind contains 2 warriors, 2 med kits, and a seismic charge.
2) LIGHT BLUE switch: 5 warriors, 2 derm patches, a 9mm pistol, and a clip.
3) GREEN switch: 3 warriors, 2 shotshells, and night vision goggles (activate the latter when your first pair run out).
4) MEDIUM BLUE switch: 3 warriors, a med kit, and pulse rifle magazine.
5) PURPLE switch: 4 warriors, 2 seismic charges, and 2 pulse rifle magazines.
6) BROWN switch: 5 warriors, 2 shotshells, a grenade, and 2 med kits.
7) DARK BLUE switch: 2 company synthetics guard the exit chamber. Make good your escape!

On HARD, you should open all 7 doors at once and smartgun the horde of warriors that emerges, then deal with the residuals left in each room piecemeal. The reason for this is that if you open the doors one at a time, you will be zigzagging from one end of the central room to the other (as well as going 'way back into the smaller rooms), constantly reawakening warriors. You will be overrun and your ammunition levels upon exiting will be extremely low. To open all 7 doors at once, activate all the switches EXCEPT the RED one that is already lit up. Hit the RED one last and make a beeline for the acid curtain and short hall beyond. This is where you will turn and make your stand with the smartgun to face the emerging wave of enemies. (You might bring a few more enemies out into the open by walking along all the sealed doors just prior to opening them.) Kill everything that comes at you, making sure no warriors get beside or behind you. Once the action slows, you can save smartgun ammo by switching to the pulse rifle and shotgun. All the enemies will be warriors except for a pair of company synthetics. This (major) feat accomplished, re-enter the central room. (True, you lose 10 armor points by going back and forth through that acid curtain, but the alternative is far deadlier!) You are now ready to go room-by-room to eliminate the remaining warriors and grab all the powerups. EXAMINE THE 2 ROOMS ON EITHER SIDE OF THE ACID CURTAIN FIRST, as this will keep you from inadvertently reviving the horde of enemies that were initially loosed when you first opened all the doors. You very likely WILL revive some of the warriors in at least one of these nearest rooms, but their numbers will be far smaller than those of the xenomorph army waiting just outside the acid curtain. The warriors move around too much for a predictable pattern in this area, but the most successful route consistently seems to be to explore the RIGHT room (as you go through the acid curtain and enter the primary room) first and then reverse your route and continue CLOCKWISE to the rest of the rooms. When you complete your circle, you will likely have to re-kill some warriors and possibly both synthetics before making your escape to the exit chamber.

HERE IS THE SEQUENCE FOR THE HARD DIFFICULTY LEVEL (activate the night vision goggles you found earlier now). Remember that the enemies move around too much for any clear pattern. This lists the enemies in each room, but the number actually present in the room (and not killed in the first big rush out the acid curtain) will vary dramatically. The room numbers go clockwise from the acid curtain, and you will explore the 2 rooms nearest the curtain (numbers 7 and 1) first.

1) ROOM 7: 3 warriors, 2 shotshells, and night vision goggles (activate the latter when your first pair run out).
2) ROOM 1: 3 warriors, a med kit, and pulse rifle magazine.
3) ROOM 2: 4 warriors, 2 seismic charges, and 2 pulse rifle magazines.
4) ROOM 3: 2 warriors, 2 med kits, and a seismic charge.
5) ROOM 4: 2 company synthetics and the exit chamber (but don't leave yet!).
6) ROOM 5: 5 warriors, 2 derm patches, a 9mm pistol, and a clip.
7) ROOM 6: (This is where you are most likely to have to kill some revived enemies.) 5 warriors, 2 shotshells, a grenade, and 2 med kits.
8) ROOM 4: Backtrack here to exit (WHEW!).

(12) Since you have already killed each enemy at least once, just make a dash from ROOM 6 to the exit chamber and simply dodge any enemies that may revive. You did it!

ALIENS: 100%.  SECRETS: 75% (???!!! Arghhh!!!).  MISSION: 100%.