Some waves are real lucky, some aren't. (Of course, whether or not you get experience is another thing.) Playing melee seems to get the job done later on enough.
- Since you usually have an open space to deal with them, Mancubus are best treated by running at them in a zig-zag to avoid as many shots as possible. After closing the gap, they're in the bag (as long as you have sub-1.00 movespeed).
- Revs are just the hella-patient game. Keep peeking out, waiting for them to show up. If you manage to get within 2-3 tiles, start running. They don't take a LOT of hits, and it's very unlikely for them to launch a rocket back as long as you're running.
- Barons...you CAN use the same ring-around strategy, although it will be more dangerous. Unfortunately, there isn't a whole lot else you can do, those guys have some strong AI when it comes to chasing. Later on with Brute3, it takes 7/8 hits, so you can try to put on some armor, turn on run, and hope for the best.
- Nightmare variants (and holy shit why did they have to be in here) are a pain, particularly the demons (haven't seen a caco yet) but a lot can be done as in the normal strategy. By now you should be level 5/6 anyway, so your build is nearly complete. Note that Nightmare Demons outrun you even with HR2, so...just a heads up.
Oh, and now that I know Arch-viles can show up, I'm not even sure I want to continue trying. It's bad enough wave 20 also had a bunch of nightmare imps, but now I have to worry about EVERYTHING respawning? If it's possible, make it so that corpses are cleared at the end of each round (and if impossible, add it to the Mod Wishlist).
It's real fun, though. I'm interested in what kind of build Gargulec tried if not what I was using. There are probably a few ways to go about it (some...more patient than mine) and round 15 is pretty impressive. For my own build, the game picks up after about round 10 and there's a lot less sniping and more luring into melee range.
EDIT: Guess I should note this was done with the previous version instead of 1.2. However, berserk armor only came in handy a couple of times, and (in some cases) didn't work at all, ala zerk armor bug.