Really cool concept! I liked figuring out the right order of cards so that you gain enough strength before attacking, but not letting the opponent attack you too much. I know you ran out of time, but I think it would have been good to make the card slots a bit bigger to show the graphics better while there are locked slots (like having the locked slots stacked to the side to show the unlockable possibilities).
The action also happened very quickly, so it was hard to keep track of whether my plan was working (it did work though haha!). Overall well done though!