As long as you do something on your turn, the character gains experience. Go to the ruins of Obel. Do not get the 6 bat and 3 skeleton map on the first floor. Finish the enemies off then gather around your highest leveled character. Use Medicine or a skill (such as Gary's Backup) on the higher level character. When you gain the ability to switch in and out, you can ensure that everyone takes advantage of this. Using a Medicine on a character just one level higher than you gains 130 experience points. The experience gained is higher if the character has a few levels on you. Once everyone is at the same level, go to Callonball and "hunt monsters". There is usually a group of monsters that is two or three levels higher than you. Once you get a character one or two levels higher than everyone else, proceed back to Obel and repeat the Medicine/skills trick. This is cost effective due to the fact that 99 Medicines cost 1980 Potch, and at this point you should be making 5,000 Potch or more per battle.