1) Beat the first 3 duels in the Fortune Cup to gain better cards.
2) Duel your way to the finals and win.
Tip: If you know you will lose a match, press the Start Button and select Exit. Then select Single Player Mode and choose to continue. You'll be at the opponent you last left off at since the game only saves if you reach the results screen.
This is very useful for when you reach the quarter-finals. Sometimes it's all a matter of getting a good starting hand and with this trick, you can retry as many times as you want.