I like this more than I was expecting. When I read the game page for this I kind of expected this to basically just be an mtg clone. Mtg is definitely in its dna, but I think you made some great changes/innovations to make your game unique and give it a ton of potential for depth and expansion.
Like others have said, you definitely need some more feedback from the game about what's happening when you and the opponents do stuff. I didn't get at first that you could play cards and attack with your ducks. I think you might have mentioned knowing that you need to balance tweak, which I agree with. I was never really in any danger of losing, and that meant that my shield and heal cards were kind of dead. And that, in turn, kind of meant that which cards I'd play every turn were mostly pre-decided.
I'd also like to call out the art. It. Looks. Great. The mana symbols and sword/shield/etc icons have a great hand drawn look. The ducks looks very good and have real personality. The trees and buildings in the background are very solid. Awesome job.
For the game jam I'd love to see like a touch more feedback from the game, a slash of balance, and another battle would be cool, even if it was the same enemy but with a few different cards or a different ai behavior, but I know that one is a real stretch for an in-jam thing.
If you expanded this and decided to make a full game, the thing I'd be anticipating that I'm most excited about is your team selection. If you can choose your party and maybe level them up or get new abilities or equipment so that they have cool static abilities or provide different mana types depending on who or what classes you have in your group, that would get me hooked. It provides a sort of list building outside of the deck building that I'd find really intriguing.