This is a very fun entry! I really liked being able to switch attacks and passives with the bosses, and it's implemented really well here as I can tell which of the boss' attacks correlate to which cards.
The presentation is phenomenal with a very unique style and incredible music.
I think the only place where the game really suffers is readability -- normally, intense graphics that clutter up the screen can be fine as the player gets used to them, but there are a few extra places wherein this issue becomes a bit more apparent. Rather than only having to look at myself and the boss, I also need to keep my eye on the cards so that I can pick them up and place them on the screen. Because my eyes are constantly darting around, I lose the boss and have to find it within the thinking time, and then prepare to dodge any attacks it's throwing at me, then look at my cards again to see what I have, all while dodging the boss' attacks...
It is, admittedly, a lot to keep track of lol. I think some easy ways to fix this issue are to maybe place the cards above the player as well for easy reach, and/or have a hotkey system so that the player only has to glance at the cards once to find out which position they're in, then hit a number to cast it and place it using the mouse.
Overall, though -- absolutely incredible job!! This readability issue is honestly the only possible issue I could see with this game, it is otherwise an amazing entry and I am blown away that you made this in only a month!!!