So, apparently, in the latest SVNs + latest official GZDoom, the bosses in ZDCMP2
get instakilled as soon as the fight starts. What's happening is that the arena is a big 3D floor over lava, and there's an "Actor Hits Floor" thing on the lava below that's meant to kill players and monsters that step on it. At some point, though, there was a change that ended up causing monsters (but not players) to trigger "Actor Hits Floor" when stepping on the floor of a 3D sector on top of it, which is not desired. Hence the bosses get insta-burned once trying to move. D:
To test quickly, start the map and puke script 888. This will place you right outside the boss arena; cross the bridge, start the match, and watch as they die immediately after stepping down onto the ring with the glass.