The idea is great, but unfortunately the game suffers from many flaws. Here's what I think could be improved! :)

Visual

Try to use visuals that match each other. Because there are power ups that are really flat and enemies have many details in their pixel art. It feels a bit off. Nothing too bad, but that's a way to polish the game.

Having larger sprites for enemies would be nice when we have to attack them, because I often clicked just a few pixels outside their hitbox. Or even without larger sprites, just a larger box.

Sounds

That's a great start, but it feels a bit empty. A drinking sound for the potion, a little pick-up option when you take the lightning to show it's a powerup you store, etc. 

Also, having different sounds for the different enemies could be nice, but that'd be the last thing to do and probably hard or not possible within the jam's time. These ideas apply for potential post-jam updates as well.

Tutorials

The game tries with the help button, but it's clearly not enough. I thought lightning would give me mana but cost me 10HP, and I was hesitating to take it at first. I've learnt part after part by myself, and would've loved to see the game guide me a little bit.

Maybe even have a recommended player to start?

Gameplay

• The game is unbalanced because of the randomness. From one run to another, I may have too much or not enough hp. Most of the time, it just isn't possible to win against the game, as no potion spawns. 
• It would be also nice that hard monsters only appear after a few moments. For example, only have slime at the beginning, with power-ups.
• There also should be restrictions once the game is more balanced. I currently can take power-ups every time I see them, and if I have potion 3 times in a row, I can recover 9hp. Maybe a limit somewhere? Or instead of a limit, a bonus if the player takes the enemy possibility.
• The ending feels a bit rough. What's this surprise, what's in it?
• Have a "prepare... fight!" warning for the boss fight? I didn't realize it was an enemy at first, I thought that its health and strength was a bonus I earned for a new run. It's midway between tutorials and gameplay.

Fun

The game has a great fun potential, but the unbalanced aspect kept me off :(

Bugs

I didn't encounter any bugs, so great job on that!

Theme

You made the effort to incorporate the theme, even if it's not the best way (it feels a bit like my game, unnecessary but here only to be in theme).

This was fun! I liked it! Would be nice to have an explanation of how the game works. Not that intuitive to change the mouse from the cards to the middle to hit the mob quickly. The program options in the windowed mode is also in .. russian? Would be nice if they were in english :)

This is really, really fun, I'd love to see a version of it with more decks so you can really customise things, but youve done an amazing job

This game is amazing love the idea. I want to see this game developed further