1- Duel them once to see what is in their deck
2- If you lose, turn off your gamebor adv.
3- Turn on your gameboy adv.
4- Go to the Status/Trunk/Deck screen
5- Change your deck to so that you have the cards that are strong against that duelist's cards.

EX: for Rex Raptor, you put in your deck Wind monster cards to defeat Earth monster cards.