Ironically, I got back into this with a friend, and we gave up around level 20 or so. At some point, and I don't know if this is a bug, but being grouped together we'd been able to fight the bosses together, but eventually it started to prevent both of us from going into the boss room, and the difficulty took a sharp increase to the point that it wasn't fun anymore. The first time we'd noticed this was with the Joker, which we'd barely beat, and we also had trouble with The Batman Who Laughs mission, but the one that made us give up with Poison Ivy. At some point, bosses become unfairly OP. I did really like the character customization though. My friend told me that he initially was disappointed that you couldn't play as legacy heroes, but warmed up to the idea of being able to create a unique hero.