Wizard: Easy experience
To easily level up with the Wizard, go to the Imperial Palace level.
Towards the end is a room with endless red beasts, and you must find
the Magic Key in it. These beasts are extremely easy to kill. Go in
to the center of the room and repeatedly use the Path Of Sky attack
(press X). The beasts will keep appearing, and you will continue to
gain experience. *** This works best with a controller with a
auto-fire feature.

XP Plateau
You'll need a third party controller with turbo or auto-function.

Stand far away from an enemy spawner so you don't destroy it with weapons. Then just simply put a piece of tape or a weight on the button for the SLASH attack and start getting lots of XP.

Auto-function controllers can be set to attack naturally (if this function is available).