If you have a Black Mage, have him or her learn the spell Toad. Then, transform all of the enemies on the field into toads. Kill any enemies that aren't able to be turned. Then, you can attack and heal your own party to build experience and JP for as long as you want with no threat from the enemies. When you're done, you can then kill the toads for an easy victory.

Addition by Verdeni:

You can actually turn everyone into Toads using your Black Mage (or whomever), killing anyone who can't be turned. Then, use the AI mode to let the battle progress, pressing the Triangle button every so often so that the PSP won't fall asleep. When you do this, the Toads will attack and do their functions without you having to participate (like the Lete River in Final Fantasy VI). You can easily get ten or fifteen levels per character in a single battle. Give it a try!