Good idea, great adherence to the theme, I felt the scaling of the cards very much when combining them. My strategy was to take a few hits and never attack while combining as much cards as possible, after a few turn I would have enough hearts and shields to survive anything and continue to get my weapons higher and higher until I could just one-shot 25 life enemies in one turn. It was very effective.
I think I won the game, I got to wave 13 and then the enemies would just come back as if it was level 1, but I had the devastantin 13 level cards.
Also the shield doesn't work when the enemies use it (that's probably why I felt the game was so easy)
Good game, well put together, it became very addictive. I wish the difficulty would scale a bit more in the later levels