I'm not sure about EXP-wise, as it's been a while since I played it last, but if you go to Jachol Cave (?) you can fight lot's of Nuteaters which give a 3 AP per fight and their grey counterparts that give like 10 AP per fight (but are basterds to kill).
In world 1 there is a monster in the desert near the libary of ancients. I think that is the best... although the forest where you get Ramth (SP?) has these little creatures that are hard to kill but give great rewards. They may take several fights to appear also.
In world 2 the Val Castle or whatever its called, there is a basement where you fight statues in groups of 2 or 5. They don't offer too much exp at all but they give good AP. If I was you I'd make sure to get all the blue magic spells in world 1 before leaving.
In World I near the Library of the Ancients you can fight Desert Chimeras in the Desert to the North. They can also teach Aqua Rake I think if you are looking to rack up so blue magics. I think I had two mystic knights, and a time mage, used haste and Bolt2 sword liberally.