Deck Options:
While in battle, press Select + Start, and then press Triangle. This will get you to the first menu. Then, go into duel. There you will see your character and your opponent, and you can mess around with both of your decks. Once you defeat him, you will go back to the game.

GameShark Codes

Infinite Life Points P1             800a18a80064
 
Infinite Life Points P2             800a22080064
 
Quick Win P1 for the Duel, Press R2 d00b4cfafdff
                                    800a22080000
 
Max Black Mana                      800a1a340063
 
Max Blue Mana                       800a1a200063  

Max White Mana                      800a1a0c0063  

Max Red Mana                        800a1a480063  

Max Green Mana                      800a1a5c0063  

Thanks to Revolution readers MangoMango Man and Gandolf!