(1) No matter how hard (or easy) you thought the first 2 levels were, you're going to be one happy camper when you leave here. You only have 60 seconds, so speed is of the essence. [BUG! I did, just once, enter a powerup level with no countdown timer going--it must not have loaded that time around.] Open the entrance door and move straight out to grab the seismic charge.
(2) Turn left to go around the corner and up the stairs--avoid all derm patches for now but grab any ammo you see.
(3) Pivot left at the top of the stairs: a secret wall! Blow it with a seismic charge. Go through the opening, grab the seismic charge inside, and blow the next 5 secret walls ahead. You'll find 3 canisters of flame thrower fuel and the prime powerup on this level: the adrenaline burst, which pumps you up to 200 health units.
(4) NOW start grabbing derm patches! Go ahead down the stairs to the exit room and get all the derm patches and ammo lying around. Ignore the exit door because you don't need to enter it on any timed powerup levels. You can also ignore any med kits, since they won't boost your health past 100--in fact, if you're good enough, you may not pick up a single med kit again for the rest of the game!
(5) Now go up the unexplored stairs and down the hall, back down the first set of stairs toward the level entrance, grabbing everything in sight.
(6) You will dead end at another secret wall at the entrance: if you've been quick, you may have time to blow it and grab another seismic charge past 2 more flights of stairs. There are other goods around (including an absolutely useless hypo pack), but this route will get you the most. Do it right and your health will be riding at 211 when your time is up.