Here's another great way to level-up easily. Have a party of five, with one of your characters playing as a Monk. Have that character learn the skill Chakra. Then, take them to a map where a "plus sign-shaped" area can be found. Have your five players stand in this area, with the Monk in the middle. Then, simply have them attack each other, using the Monk's Chakra ability to heal everyone each round. This will rack up some easy JP and experience points if you have some patience.