(1) There are so many goodies here this is almost a powerup level, but you still have lots of enemies to do away with. I recommend using grenades or seismic charges to kill the company soldiers in areas where you will be retracing your steps, simply for convenience. Enter the level and grenade the 2 soldiers directly ahead.
(2) Directly to your right, just outside the entranceway, is a secret wall hiding a seismic charge. Ignore the door opposite it for now--you won't be able to get in until you are nearly finished with this level.
(3) Move forward and take the first right. Blow the crate and grab the battery. In the room ahead are 3 soldiers; after killing them, grab the clip, derm patch, shotshell, and auto-map. Don't activate the switch (to turn off some fire jets) just yet, because it causes 6 facehuggers to materialize outside the room; leave them for later and you'll only have to kill them once.
(4) Turn right when you return to the primary hallway. There is a door on your right, but ignore it for now to avoid re-killing more enemies later. Kill the soldier ahead by lobbing a grenade over the crates he is hiding behind. Directly behind him (at the corner) is a secret wall, but you will want to put this one on hold for the time being, also. Now move toward the piston lifts. Carefully sidestep in front of the switch to your right to avoid the fire jet and activate the switch to turn on the 2 pistons directly in front of you.
(5) You can now move ahead onto the 2 pistons and hop to your right into a new area of the game. Be careful whenever you are on the pistons as any fall is fatal! There is a soldier waiting when you arrive. Kill him and move ahead.
(6) Take the SECOND right, but don't miss the secret room in the corner ahead where you will find a med kit and seismic charge.
(7) Proceeding down the unexplored hallway, blow the crates on your immediate right. Flip the switch and open the now-exposed door. Shooting the crates yields 2 med kits. Hold on, though--you ain't done yet! Once in this room, face the left wall. You can blow it (plus 2 more dead ahead) to reveal a total of 3 shotshells, 3 flame thrower canisters, 3 pulse rifle magazines, a seismic charge, and a grenade! A single soldier will come after you about now; eliminate him.
(8) If you can still walk with all the goods you are now carrying, move back out and down the hall. Blow the wall just to the left of the fire jets to reveal a very narrow secret area housing 5 facehuggers. Don't worry, though--normally, only 1 or 2 lucky winners survive the blast and need dealing with. Now you can either watch your timing and run past the fire jets or avoid them by walking into the now-revealed wall recess; watch for acid from the 'huggers, though.
(9) Open the door ahead and kill 2 soldiers. Blow the barrels and crates to find a battery and night vision goggles; the exploding barrels will also blow a hole in the wall. But wait--the new wall opening is a trick to get you to leave without all the goods!
(10) Throw a seismic charge at the wall behind the row of crates you opened and inside you'll find, either lying around or inside crates, 2 shotshells, a med kit, 3 flamer canisters, a pulse rifle magazine, a seismic charge, and 2 grenades.
(11) NOW head back to the hole in the wall. In this room you will kill 4 soldiers (one coughs up grenades) before taking your time to grab all 7 derm patches lying around on the tables and desks.
(12) Now approach the far left corner of the room (you will pass a switch to your left) and blow open another secret wall ahead of you and behind a pillar. Within you will find 4 eggs, a seismic charge, and 3 med kits. Activating all 4 switches in the main room will turn off some fire jets, activate another piston lift, power up that door right near the entrance, and open the exit doorway. One switch also causes 2 young dogs to materialize near the hole in the wall that allowed you access to this room.
(13) Dispatch them before opening the door here and killing the grenade-toting company soldier right in front of you. Move forward into the hall. The crate to your immediate right, as well as the 2 just ahead, each contain 1 facehugger.
(14) Continue forward and open the door on your left (after blowing the crates in front and killing the pair of 'huggers that pop out). Grab the battery inside and activate the switch.
(15) Emerge from the room and continue left. The wall on your immediate left can be blown up to uncover a flamer canister, shotgun shell, and night vision goggles (I don't know where on this level you really need the goggles). A company soldier will make an appearance about now; don't keep him waiting! (At this point you can run up and down the hallway to repeatedly resurrect the 2 grenade-carrying soldiers behind you if you are running low on these explosives. This works best if you grenade all the other soldiers in the area as well as the 2 dogs beforehand. And, of course, you can't use a grenade or seismic on the 2 dudes toting the goods if you expect to successfully reanimate them later.)
(16) Now stay left all the way back to the pistons. Two newly-materialized soldiers will come after you across the lifts. After killing them, move straight ahead across the pistons and then right to get to the exit area. Throw the switch to activate the final piston and allow you access to the last part of this level.
(17) Now stay ahead and to your right on the pistons to a very small area. Looks pretty sparse, I know, but behind the secret wall right in front of you lie 6 facehuggers and the prime pickup of this level, the smartgun (Oh, yeah!). Kill the baddies and grab the goody. You are now officially ready to rock!
(18) Ignore the already-activated switch here (function unknown) and stay right on the pistons to get back to the first part of the level and the numerous unopened rooms you left behind. Upon arrival you will have to kill any revived enemies as well as 2 new young dogs.
(19) First, run into the room that had the 3 soldiers in it and activate the switch you left untouched.
(20) When you near the doorway, 6 'huggers attack. Kill them and head for that unopened door right next to the entrance. Behind it are 5 patches and body armor.
(21) Now move back down the hall and open the last door on your right, but be warned: between you and the piston lifts lie 2 freshly-materialized young dogs plus the pair you killed here earlier, and you will probably encounter a couple of them now. Inside the door are 4 eggs and a seismic charge. The wall at the end can be blown up. Careful--out come 2 of the fastest-moving young dog aliens in the game! Two med kits are in this secret room if you need 'em.
(22) Now head back toward the pistons, dealing with any remaining young dogs along the way. Don't forget the untouched secret area in the corner. Within are a seismic charge and 4 crates containing 3 'huggers and a single derm patch; behind the crates lie another facehugger and an adrenaline burst. If you walk over the adrenaline burst and it doesn't disappear then you're doing okay. At this point a newly-appeared young dog may try to get at you across the piston lifts. If so, it is easiest to wait for him here. Otherwise, you will have to seek him out where you found the smartgun and try to kill him without getting 'hugged by one of the facehuggers you left behind.
(23) This done, you can now get yourself and your shiny new smartgun to the exit doorway.