alright, I made it by your description "Cards have HP and an ATK, they take turns attacking and the first to reach 0 loses."
you can customize and add onto it, other wise it's pretty boring
currently theres 2 cards in the player library, and enemy players can only use one card. You'll have to add in another event branch if you want enemies to use more than one card.
currently the player can hold as many cards as they want. To add in more cards to the library just repeat the conditional branch process, eventing is pretty crude, but it's the simplistic crude way.
There are better ways to create bigger card systems, but this is pretty much just so you can see and try to learn from.
http://www.mediafire.com/?9y2xjyym1mt
talk to arshes to play him in a dual, talk to the girl to get more cards.