You've got a solid game. I like the visuals; my only complaint would be, and I know it's part of the style, but maybe the character and cursor could be just a bit more complex or colourful? It would be good if the cursor could stand out among the chaos during fights.
Question: is there a time limit with the last card exchange screen?
I liked the card system, but I think it would've fit the "exchange" theme more if they were further connected with the fights, rather than being in between. Maybe they are scattered around the arena, or you trade them with the boss?
Other than that, the bosses were challenging, but didn't feel unfair. Their arenas and design fit the elements theme you were going for. The phases, variety of attacks, and patterns kept it interesting.
I had fun, thank you for the experience =]