This was fun to play! I love the art style a lot, it's really neat. The mechanics of the game are pretty solid too, but the one thing I think you could improve on is teaching the player in a better way.
I normally dislike reading long tutorials and like to judge a game on how well it teaches me how to play without the use of an external guide, but I had no idea what was going on initially and had to refer back to the tutorial a few times before I understood it. I appreciate the effort to make the tutorial look pretty though!
The layout of the game is a bit confusing too. I think too many similar elements are spread across the screen and don't really correlate well to each other. Going from under the boss image, telling you how much mana you need, to your card in the center, to the top right seeing how much you had, could all be done in a much more compact and readable area, in my opinion.
Very well done though!