to add on to TeenGogeta's point, world 5 with the mummy bosses, coupled with the undead mace, meant an easy grind to max level in 15 minutes (as well as farming millions), because the mace only kills underlings but not bosses. It ended up being hilariously fun killing hundreds of mummy spawns at a time while 6 mummy bosses kept on regenerating more. Use mice as fodder to avoid accidentally killing the mummy bosses.
another suggestion: make the ending skippable. other than that, no complaints. good hours wasted.