Yu-Gi-Oh! The Dawn of Destiny Cheats

After you beat all fifteen level 3 tiers, view the credits then go to single duel mode and hold X. While holding X pick the character whose deck you wish to use, then let go of X. When you start your next duel you'll be playing with whoevers deck you chose. It's also a good way to see what cards are in the other duelists' deck.