Out of curiosity, what's the "expected" way of dealing with map 21?
My method was:
spoiler - highlight to read:
Sprint to the Berserk room, hit the switch, grab the 'Serk, run. Go up the bridge, grab the Soulsphere and the Shotgun; kill the Chaingunner, upper-ledge Revenant, and Mancubus with the Shotgun, kill the Imps with the pistol, punch out the HK, the 2 Spectres, and any Pinkies that are wandering around up here- make sure to do it on the opposite side of where the Vile is going to spawn in. Grab the box of bullets, hit the switch. Enter the cave, punch out the Pinkie, HK, Spectre, and Cacodemon. Grab the chaingun, hit the switch. Punch out the Revenants, carefully baiting them across the bridge one-at-a-time if possible. Take the teleporter, hit the switch. Carefully snipe at the Vile with the Chaingun, using the candle-pillar as cover from him and the rock pillar as cover from the Arachno. Finish him with the Shotgun after running out of bullets. Run through the rest of the level without killing anything else unless you have to punch out an Imp or two that's directly in your way (the guys in the Berserk room, the Arachno, and the final spawn-in on that ledge survive.)
Finding no secrets probably didn't help things any.
Also, Map 24- I haven't finished this one yet (best attempt yet was killed by the Cyberdemon), but the enemy placement in the Yellow Key area is bizarre. A few Lost Souls that can't attack you through the impassible lines, then a trap of Pain Elementals that can't attack you because of those same impassible lines? What?