During random battles (not story-driven battles), getting experience and JP are vital to the growth of your characters. Since enemies in random battles can almost always be handled rather easily by your party (as they grow with the strength of your party), you can eliminate all but one member of the enemy party rather easily. Leaving that enemy alive, you should then have your characters attack one another, healing with Chemist skills and White Magic if you can, until you can no longer survive without killing off your party. Then, go after the final enemy and eliminate it to finish the battle. By doing this, you can create experience and JP you wouldn't have gotten in the battle otherwise, lessening the amount of random battles you'll have to fight to build your characters' levels.