Loved this game. It was pretty straight forward even without a tutorial (and that's coming from someone that isn't that familiar with deckbuilders and card games). The artwork is crisp and the overall flow of the game is pretty satisfing. I have two thoughts though (not about balancing the items. I get that it's a demo and it needs playtesting):
1st: I think that, compared to the rest of the game, the boss battle lacks depth. It was pretty much "kill the boss in that amount of turn, or you are dead". It felt a bit flat and anticlimactic, especially compared to the rest of the game were i was constantly making dicisions on which cards I should keep, in what order I should use my weapons, what stats I should prioratize over others etc.. And the boss battle was pretty much like: open the cards -> take the weapons -> kill the boss. I was pretty intrigued by the choices I was forced to make in the first 29 levels and I would love to see those tight decision making scenarios being pushed to their limits and implemented in the boss fight. (And I know that that's just the problem and I am not offering any solution to it, but yeahhhh.... just my thoughts here)
2nd: That's not so much of a problem per se, but it's just something that (always in my opinion) would make the game more enjoyable. I believe that some particles, white flashing and even some very slight screenshake would make the impact of your actions (attacking/taking damage etc) feel better. But that's just me. I understand that some people might find that distracting. So yeahh Idk ¯\_(ツ)_/¯
Overall, it's a very nice game! I enjoyed it a lot. And considering it's just the demo, I can't wait to see the final project! I am surely wishlisting it on Steam! Keep it up!!