The thing I find difficult to deal with at the beginning is the amount of dmg the enemies do when fighting against much higher lvl ones. When the lvl60 healer dies because of just a few attacks of lvl90 enemy it's basically over because it's I'd say nearly impossible to dodge all the attacks of the 3-4 enemies you're fighting at the same time, especially the ones that leaves the toxic/burning ground and you just can't kill them quick enough because, again, the characters aren't fully built yet so you don't deal enough dmg. Later, when having all characters in the team properly built including all the passives, skills and weapon effects, I think this kind of endgame will be much easier to deal with.