There are two spots that I like. Just northwest of the docks to Lindblum there is an island that has Grand Dragons on it. They are weak against Ice and cast a lot of Thunder so use coral rings or rubber armor to absorb or reduce the damage from the Thunder attacks.
The best experience in the game is from fightings Yans on Vile Island(it's southwest of the Ilfa tree and southeast of the Shimmering Island). They are hard, but give a but load of experience. I would suggest fighting the dragons to get everyone in their 50s and then trying your luck with the Yans.
