Yojimbo kills everything, if you pay enough, even Penance. But of course, you should try "proper" way, it's much fun.
But first, I would go to Monster Arena, buy catching weapons and start to catch monsters. You got lot af items as rewards, and before you catch 10 of every monsters of every area, you got lot of experience. Then, open last dungeon (and catch monsters there too).
Now, you will be probably grinding to kill every side-bosses in the game, like Dark Aeons and Arena bosses.