THE GOOD (what i liked, and what stood out for me):
A very, very creative game with a nice mix between making your own amazing (sometimes monstrous) musical creations. I never know i could play the bagpipes and drums at the same time XD. I also quite enjoyed the artstyle, storyline/setting and the card-themed aspect to it. Being "joined together" with your enemy was a good decision! Enemy design was superb, I felt like I was going against a musician who really, really wanted to get laid by a princess and only barely managed to beat him.
THE BAD (what i disliked, the bugs, and other problems):
Unfortunately, understanding the game was quite hard, and for me it felt like the card combinations were completely random (apart from bagpipes and drums. DONT do that). The melody was also slightly dissatisfying, since the tunes were always off tune to each other even if you get a ridiculously high score such as 100+. Having some harmony would make things more satisfying. It would be a good idea to include some labeling to your UI, such as to that of the enemy or indicating that something is a "health" bar.
THE HARSH (stuff that would just make the game better, usually just personal opinion, don't take it too heavily :D ):
I felt like the game loop was a bit repetitive and a bit too slow, maybe make it so that you can deal more damage!