After completing the game, save and then use that character to complete a castle. Press STARTon controller one, then hold UP + A on controller two. Save the game and start game play with a new character. The new character's experience points will increase quickly after starting.
Alternatively, complete the first quest and go to one of the palaces during the second quest. After he has placed the crystal in the statue, his experience points will increase rapidly. While this is occurring, press Up + A on controller two, select the save option, and then resume game play with a new character. This new character will also gain experience points quickly.