Eye of Cthulhu
and I meant eater of worlds
I nearly defeated all of the bosses in expert mode, except for the moon lord. In all of my playthroughs, the moon lord was hard even with the nurse strategy. And now it doesn't even work because now the moon lord can shoot eye beams through blocks so you'll most likely need a rod of discord, and I don't even want to start talking about the tedious task of trying to get it.