The game is nice and the art is very cute.
I would recommend rethinking slightly the controls.
Most players are used to move with WASD not with arrows, and while moving with WASD, Z and X are quite uncomfortable. The spacebar for jumping is fine, I would recommend J or K for shooting.
Also I'd have the enemies be a little less aggressive on the first wave and have them get more and more aggressive as the wave proceeds. This, together with the raw enemy count, can ease players into the game.
The countdown being the cards and the wave was a nice idea!