Go to the catacombs at any level (level 1 recommended). Have all the characters that you want to gain experience and a healer (required). When the battle begins, attack your own teammates. After the attack, your character will gain experience. The higher the character's level that you attack, the better. You can either heal the damaged character (for experience for the healer) or kill him or her (much more experience will be gained). Note: If your characters are killed, their Luck status will be reduced.