Danger Mode gives the bosses slight HP regen, in exchange for them having less health.
Boss 11's second form's pinch is probably the most annoying thing in the game, and is really bad in the arena (I suppose I can just make him skip that phase for The Arena). The idea is that he's throwing out all of his attacks as a last resort, and the player is supposed to calm down and watch each attack carefully, but I guess it's pretty counter-intuitive that it happens at the very end of a fight that is generally a "HP melter" so you won't have much HP at that point anyway (Including getting reset to 5 or 6 from a bug I'll fix lol).
IDK what happened with Bohboh though, clipping through the wall seems very strange, and the press ESC to die thing shouldn't ever not work while the hold ESC to quit does, so I'm still trying to figure that out.
Good luck with The Arena, and once again, sorry for the bugs!