This trick can be used at any point in the game. Buy the Amulet from the magic item shop. This may seem like a waste of money, but has a special power. If you are about to raise a level, use the last charge (third use) to kill the monster which would of raised your level. There should be a one-second delay followed by a flash of light. If done correctly, your level, gold, and ring power will be maxed out.
Adjust power level
During game play, enter the status screen. Hold A + B + C on controller two and press Up or Down to adjust the power level.