Had a lot of fun, just a few comments.
- This could just be me being dumb, but it wasn't clear to me that cards that do effect damage can target players. This led me to not those cards when my opponent had no monsters on the field because I assumed I would be forced to target one of my own monsters if summoned since my opponent would have no valid targets.
- There is some jank with the "boss" stages. It seems they get to go first, since it says enemy turn, but then they don't do anything, but I'm able to play cards. Then once I play a card they do their turn like normal and then it goes to my turn normally. This happened every time for me so I'm not sure what the steps are to reproduce the issue.
- I feel like there should be some more visual distinction between the spell and monster cards besides just the border. I think it would help if the spell card art wasn't just characters standing around like the monster art. Not sure about other card games but in yugioh a lot of the spell/traps focus more on inanimate objects, characters doing specific actions, or action scenes (though there are many exceptions) which I think could help set them apart from the sea of AI generated fantasy character portraits.
- UIs are very nice and seem quite polished
Overall, keep up the good work!