Nice concept. The pixel art and music is good. It is quite challenging even at easy difficulty.
At first it was confusing because we usually drag and drop cards in card games, and when I do clicked the card, I don't know the next step would be to click a blank space under the enemy card. It would be nice to have a drop zone indicator when you select a card to signal the player "you can place it here".
The flow is easy to understand but it needs balancing. For example in my first session, after level 1 I can choose shop or spar, I chose spar (works like merging), then at level 2 I faced two Walls that obliterate me easily because I ran out of cards not enough damage.
Overall, I think the entry is great and polished well within 10 days. Well done.