After you beat the game and defeating Mr. X, you will unlock the "Duel Room" at the main menu.
Get rare cards quick
To get rare cards early, you must do a lot of special trades. Save before you do though, so you can restart if you lose. If you do not see any rare cards in the selection, just choose a card that you don't have. If you win, you will get to choose different cards. If you do this in every town, you will have more rare cards than you need. It will also make you much harder to beat.
Keep your cards
Early in the game, a gypsy will take your deck and swap the cards in it for weak cards. You cannot go around her, and you must go past her to enter the tournament. To keep your deck, swap all of the cards in it for cards in your trunk that you do not like or need. Walk past her, and after she swaps your deck, switch all of your original cards with the ones she gives you. This allows you to have an easier time beating the rest of the game.
Recommended Deck
When you choose your deck, choose the Fire Deck. Later in the game you will get Bollshack Dragon.
Special Cards
In a trade shop, save and quit. Select "Continue" and in "Special" you should have different cards.
Duel Room...