Cool dynamic card game! I actually thought it was fun to play (but I had to play on difficulty 1, R was too hard). It wasn't clear to me why sometimes I wasn't allowed to use any more cards, it's like even with card cost off there was some sort of limit on how many I could use. That, or maybe it was the enemy's turn without me realizing? I later turned card cost on just to see what that looks like because I thought I was somehow still energy-bound even with card cost off.
Would be cool if the game had its own graphics and music! I understand you probably just took images from somewhere to get a proof of concept going.
When I made it to the final boss, the place where you have to drop cards on him wasn't where I expected it to be and I kept missing it at first. This resulted in me having a plethora of cards, at which point I realized 'Claw attack' and 'Shield' looked identical when you have too many cards, so I never knew which one I was grabbing.
Overall, I think it's fun to play and has potential. Nice work!