I was recently playing Castle Crashers with my two roommates. They hadn't yet finished the final boss, but my character had. We went through the last few levels that they hadn't completed, making sure not to skip anything. After completing the game, they didn't get the achievement and we couldn't enter insane mode. Is this because of some sort of level parity, or are you simply not allowed to beat the final boss if anyone in your party already has?
They created a game without me and were allowed to go directly to the final boss and after they beat it without me, everything unlocked as it should.