The way to get easy exp. is to keep fighting Wakka on Sora's island. Everytime he throws the blitzball at u hit it back at him (it gives you 1 exp. and if you hit him you'll get another exp. point) and when he does his powerup shot hit it back at him (it gives 2 exp. and if you hit him you'll get 2 exp.).Watch out though Wakka will eventually give up then vs. him again and repeat (it takes a while but it is worth it especially if u wanna beat Riku).