To get easy experience for your lower level characters, go into any battle and order your lower level character to attack one of your own characters that has a much higher level. When the attack meter appears, press X at the lowest point so that the damage done is not too harmful. The amount of experience gained depends on how large the difference is between the two characters. The best place to do this trick is in the first set of levels of the Tower Of Trial, where your opponents should not be dangerous.