The only thing that has currently stopped my progress was a pair of archangel bosses that immediately turn all your monster's HP into 1 at the start of the battle, something I have to find a way to deal with yet, but I could prob win by sheer luck just by having my allies be faster then most enemy monster and have them heal and ressurect eachother before they all die.
Best bet for something that doesn't involve that much effort is probably hunting up one of the action order manipulating traits, raid or somethin'.
Personally wrecked 'em so far with that DoLIA and a griffin team that's built around jacking barriers through the roof (that happened to involve a lot of healing, as well, and a passive defense stacking/equality feedback loop that'd easily end up with some of the griffins with five digit stats by turn two or three... and their defense equality was pinging off of started around 200, at the time). Did have to have enough speed to get in at least one turn before something blew up the entire team (mercy or otherwise), but that's not
that bad, usually. Tend to have at least one or two creatures spec'd for speed anyway. Want to say there was a third one, too, that didn't do anything in particular that was special besides hit hard and fast -- starting death ghoul, coast watcher, stuff like that. If mercy's dead before it can act things get a lot more reasonable, similar for everything else, ahaha.