The sprites are super cute, and the core game loop is a lot of fun. I liked the progression in difficulty too. The first round serves to teach the mechanics of the game, and then the opponents get tougher.
My only criticism is that the theme didn't really come through. It would have been nice to see the cards themselves morph in some way. Especially since they're based on elements, something like combining cards (trading two for one as a gamble) could have added additional challenge.
But the gameplay was already a great take on the rock-paper-scissors concept.