Go to the basement of Val Castle and fight statues. They come in groups of two and five. Two will give you 4 AP, and five will give you 8 AP. You can kill one instantly by giving it a Soft, or you can just use Lv.5 Death to kill them all instantly. Another place is the Phoenix Tower's magic pots. They each give you a whoppin' 100 AP, but you need to give them an undetermined amount of Elixirs first. The magic pots aren't really random encounters, so you can only run into them a fixed amount of times. Also, elixirs are rather hard to come by in this game. So I suggest fighting statues.