[10,000 Volt Ghost]

Even with the mastery ring it was taking forever so I checked out survival mode. Basically you equip the item that uses HP instead of MP for skills then just burn through it using the one you want to level. When you die you respawn with full health. Rinse. Repeat.

Also select the main subweapon you want to use as DOWN B, then use it while hitting LEFT BUMPER. It will spam the attack faster so its easier to level up. Still takes forever though.