(1) Your first task on this level is to acquire the acid vest unscathed. Run out the entrance door and take the first right, dodging past the security guard covering the hall. Stay right around the pool and up the stairs. You may have to try several times to get this far without taking a 9mm hit from the guards.
(2) Activate the switch at the top of the stairs (it turns some lights on and causes the acid vest and 4 facehuggers to materialize in the pool). There is also a med kit by the switch.
(3) Run down the other side of the stairs and jump in the pool to grab the acid vest. You may have to shoot a facehugger or 2 before dipping in. With 100 armor points now protecting you, finish dealing with the facehuggers and the 3 security guards that followed you into the pool area. There is another med kit at the far edge of the pool if you need it. Now go back straight across the main hall and shoot the single guard behind the stairway and the pair at the top of the stairs.
(4) Stay to your left until you are facing a wall and have crates on your right and left. Blow the right hand crates and kill the facehugger that pops out. Now blow the crate behind the barrel on the left-hand stack. Lure the warrior revealed to the barrel and blow the barrel to terminate it. In the enclosure you'll find a single 10-up derm patch. During this time, 1 or 2 facehuggers may be activated in a nearby room and come after you. If you see them pop up on your tracker, go back the way you came and look down the ramp on your left (it has steam venting into it). Kill each with a single burst of the flame thrower. Now return to the warrior you just killed and face the stairway on your right. Three of the 4 crates forming the next barricade contain facehuggers. Blow the barrel and you release them en masse; blow the crates one at a time and the critters are more manageable. If you open the crate directly behind the barrel last, the 'hugger inside will remain behind the barrel and can be killed safely when you blow the barrel up. Now walk up to the next barricade and break open the crate directly behind the barrel. This will release a facehugger and lure a warrior up to the barrels. Wait for the facehugger to run back near the barrels and then blow both the aliens up. Approach the doorway ahead on the right and back up as soon as your motion tracker starts beeping; the warrior emerging will take only 2 shotgun blasts to kill (was it partially damaged by that last barrel explosion?).
(5) Inside the room, throw the switch to turn off the steam valves at the ramp and grab the shotgun shell. Now go back the way you came. You will have to re-kill the 3 guards at the stairway.
(6) Next, face the recess in the wall to your left just short of the ramp with the steam vents you turned off; it looks like a row of blue lockers. This is a secret wall. Poke your 9mm around the corner and spray the wall back and forth with bullets. In a few seconds the wall will come down, releasing 2 warriors. Switch to the shotgun and start blasting as they come at you, continuing as you back up around the corner and back toward the dead security guards. Rarely, 1 of the pair will jump up on the railing to your right and run on it as it comes at you! Once they are dead, you can enter the secret room to grab a shotgun, flame thrower, and shoulder lamp. Now descend the ramp with the shut-off steam vents, staying to your left and grabbing the battery and shoulder lamp on the way. At the opposite end of the hall you will enter there may be 1 or 2 facehuggers waiting if you didn't have a chance to kill them on the steam-vent ramp.
(7) Before proceeding further up the hallway, blast the left locker for a shotgun shell and seismic charge.
(8) Continue on into the room ahead and kill the security guard. There is a 10-up derm patch and the auto-map sitting on the beds. Turn around and face the lockers you passed when you entered the room. The second and fourth ones each contain a 'hugger; release them one at a time and kill them. You may have to chase them down the hallway, but be quick--you don't want to follow them so far that you start running into old friends! The third locker yields a shotgun shell and seismic charge. Now go to the other end of the room and activate the switch to power up the door right in front of you. Open it and start firing at the 2 security guards on the other side.
(9) Once the guards are dead, open the door on the left. A warrior and 2 'huggers will rush out; try to kill them back away from the door. Now enter carefully and try to lure the 3 remaining warriors out one at a time. Be quick to make sure the door doesn't close before you can exit it with a warrior hot on your trail! Killing these 6 enemies outside will save you from dealing with them again--plus another trio of warriors--later. Sometime during this process a new security guard will appear behind you from the far end of the corridor you are in. Kill him and run down the corridor to the entrance door; there will be another recently-materialized guard at the bottom of the short stairway. Kill him and the original (now-revived) guards that come after you.
(10) Now go back to the other end of the hallway, enter the room, and grab the 2 5-up and 5 regular derm patches off the central tables. DO NOT go up the ramp on your left yet!
(11) Go straight down into the switch room and throw all the switches. These will turn on some lights, open the exit door, and activate a lift. Save the lift switch for last: it is the one on the left at the far wall as you enter. AS SOON as you hit it, sidestep right and run forward onto the dropping lift; if you are too slow, you will lose 15 health points (no matter how much armor you have) due to the fall.
(12) Once down, throw the switch on your immediate left now before you forget, or you will regret it! In this room are a large number of crates with various goods. The second on the right as you enter contains a facehugger; I like to blow all the other crates first to give me a little room and release it then. Now activate a shoulder lamp and start working your way up the facehugger-infested ramps. Most can be lured from around corners by running forward and then backing up (with a clear view ahead of you) when they come after you. There will be a total of 8; fry them all with the flamer.
(13) At the top of the multiple ramps a door should be open. If it isn't, go back down to the lift and activate the switch you forgot; enjoy dealing with all the revived 'huggers on the trip down and back!
(14) Enter this doorway and kill the 3 warriors within. You are back where you started before dropping down the lift.
(15) NOW go up the ramp on the left and kill the warrior waiting there. Ahead on the left are a med kit, shotgun shell, and flame thrower canister behind one of the trickiest steam vents in the game. The first vent can be snuck past by hugging the wall. The second steam vent is hard to figure because there are 2 vents steaming alternately. Watch carefully to get the tempo and dash quickly when they both subside simultaneously.
(16) Go back past the steam vents the way you came and continue to the exit door.