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 grabbing XP. Auto-function controllers can be set to attack naturally (if the function is available).
Unlimited lives
In two player mode under the normal or greater difficulty setting when you are down to no lives or are dead, press Start and choose to leave the game. Then, press Square to load your character. You will be back at full lives. Note: This will get rid of anything obtained since your last save, such as money, upgrades, etc.
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. Note: This works best with a controller with a auto-fire feature.