A story excuse could be provided - he was stuck in limbo for 50 years while Damian was out building his empire.
And who says that you always need to start out at level 1? This is a pre-existing character, you could start him out at level 20 and facing level 20 enemies to start out with, and Damian is level 70. Is there really a difference between starting at level 1 and facing a level 50 final boss?
