This is an excellent idea. I love the premise of having to quickly decide and act upon what spell you're given, though at the moment it feels quite unbalanced and luck driven.
The green enemy in the second battle turned out to be unbeatable for me as I kept only getting shields and the occasional fire spell was too weak against its healing. When I replayed to try again, the second battle kept spawning more than 5 enemies, which was impossible to fight - is that a bug or was it intended? Another (probably) bug I discovered is that you can link an enemy to itself to do double spells by clicking two times with the link spell.
The shop between rounds was confusing because I couldn't see how much money I had, so I had to guess and click until I couldn't buy anything anymore.
The art is clear, simple and beautiful, the sounds are very well picked, but I was hoping to hear some music in the game, that would've made it much better.
Overall a really nice game idea, I could definetly see this going far!