This will need to be done on your second (or third, etc.) play through the game. After you finish the game, start an EX New Game and buy the perk "10x experience" from the Grade shop, along with anything else you can afford that will help such as carrying items, artes and skills over. This way you will level very quickly and can overpower all your enemies and bosses. Be sure to skip any dialogue possible, avoid random enemy encounters (use the item "Holy Bottle" often) and only focus on the main story missions.