I know Rapps was an optional boss, but I thought he was super hard... I just kept dying with Aero 3. Anyway, of the not optional bosses, I'd say Carry Armor and Lost #. I died a few times with each of these guys. And Materia Keeper was no cake-walk either: TRINE SUCKS! For a lot of stuff I just use the Big Guard enemy skill and wail on them while I can and when it runs out, cast again if need be. Its seriously been a life saver on many occasions.