You probably want to reach this before facing the final boss. it's shouldn't be much of a issue especially not if you going for the rare item drops required for the hidden summons and onion items. if you follow the naming way side quest you will get a XP boost augment when talking to him at his home on the moon.
Tower of Babil or any of the deeper caves on the moon will good for a lot of high but a bit tough XP battles.