(1) Enter the first circular PILOT'S CHAMBERS, watching for the 2 facehuggers that will beset you the moment you move out. Use the pulse rifle or shotgun, since those are the 2 ammo types to be found on this level. Grab the 2 derm patches immediately ahead and on your right and begin working your way forward in a clockwise direction.
(2) Near the next pair of alcoves are 5 more patches and 1 'hugger.
(3) Move on to alcove pair 3 for another 2 derm patches, and watch out for the facehugger that attacks here. Now be careful! The left alcove here hides a secret room containing 3 facehuggers (as well as acid boots and a med kit). The trick is to avoid them PLUS the 4 newcomers (more 'huggers) that materialize directly behind you the instant the door fades. This is one exceptional situation where you may want to use the smartgun on smaller enemies. With it in hand, approach the left alcove. As soon as the secret doorway begins disappearing, fire at the 'huggers behind it. Ideally you want to eliminate all 3 of them in 2 rapid volleys. This done, dart forward to don the acid boots and pivot to face the corridor. If you have a second or so to spare you can switch to the pulse rifle to deal with the 4 remaining facehuggers, but be quick because 1 of them often tries to leave the area.
(4) Having survived this onslaught, move forward to alcoves 4. The right recess is the exit chamber, holding 2 shotshells and a med kit; you can reveal it, but do not exit yet. The left alcove houses a pulse rifle magazine and another med kit. If your pulse rifle is near to capacity, don't grab this powerup yet. If you need the ammo, take it, but as soon as you do so 3 facehuggers will appear in the main corridor; kill them and move on.
(5) There is nothing at the next pair of recesses, but in alcoves 6 the left-hander houses a med kit. Back into it so that you are ready to deal with the 4 facehuggers that appear when you get near it.
(6) Kill them and continue forward all the way back to where you started this level, grabbing a single shotshell and derm patch en route.
(7) Back at alcoves 1, you will meet some old friends plus 1 newly-materialized facehugger. Same story at alcoves 2, but with 2 new 'huggers to deal with. Continue killing revived facehuggers until you arrive at the exit on your right in alcoves 4. Don't forget to grab the pulse rifle magazine (and kill the 3 facehuggers that come with it) if you left it behind earlier.
(8) Now exit, being sure to grab both shotshells and the med kit (if needed) in the brief second you have to move around in the exit chamber before the level actually ends.