Breath of Fire IV

Sinchon. Face Bots (yellow ones, not the red ProtoBots). Do an elemental combo attack, and they give double the experience, as well as burst open, leaving them defenceless. Three turns later they'll go back to normal, which is when you can do another elemental combo attack, doubling their exp worth again. Use the two turns when they're defenceless to heal them.

You can get up to 65535 (the absolute maximum per enemy) exp per bot this way. As you get stronger, you may wish to use Barrier to reduce your damage (as well as use the eath+fire elemental combo attacks in place of the others due to the Bots resistance to earth). It will eventually get to the point where they will die even with these preparations to extend their longevity, but by that point, you should be strong enough to handle the endgame enemies.