Cool game, played longer than I thought I would
It's a bit weird that mana is heart shaped, should probably be star shaped
I think every card should cost mana as I never had to look at the mana meter when casting cards
I think the hitboxes are a bit off, seemed to not take damage when I should have a few times
It's also not great that the cards can't combo with each other since that's one of the most important features of a roguelike like this
There's a bit of an inconsistent artstyle in this game with high quality pixel art, minimalist pixel art, smooth drawings. Being consistent is more important than having high quality art. The reward coins also need a proper sprite other than just a circle.
Not sure about getting every card at once between waves as well, I feel like they should either be earned through dodging or collecting, or each have a cooldown. Or if there's a mana rework then having mana collected on the battlefield or have a cooldown would be good.
A small change that would be nice could be making time only slow down and not stop when choosing cards to keep some of the tension.
I didn't seem to get a reward from beating an elite enemy (skull with crown). Seems strange, players should be rewarded for more difficult fights.
Also not sure why I lost all my mana/health upgrades after beating the boss, not necessarily a bad thing but should probably be communicated to the player at least.
I will say that this game was more enjoyable than One Step from Eden for me. I really like the merging cards mechanic and all the charming characters. Combat felt pretty good in general as well. I recommend you increase the player shot speed, add particles, bassy sfx, knockback, more particles, etc to make the game feel super juicy. The "Juice it or Loose it" talk on youtube is great for tips about this.
All in all, great work!