Here's a way if you want to level up fast.
First, you have to own the Gem of Will (from the 2nd ToV at Sakai Facility[backtrack]).
Continue your game until the last stage. Gem of Will will hold your HP to 1, so its hard to battle the bosses. So, just fight all the bosses until you're about to fight Fortinbras.
When you're fighting him, spare him a little life at the end, then equip your Gem Of Will and quickly use Ultimate Critical [tap triangle] to kill him. You'll get easy 500000 experience. You can use the same way at Fortinbras[Human Form] for more exp.