I'm officially addicted to the gameplay. And guess what, I think I've figured out the trick, because I've managed to beat it multiple times! My little riddle is this: Horns are blown in pairs, Barbell and Poisoned Edge are essential, Backpack makes everything faster, and Scout absolutely loves Ranger.
There's definitely a learning curve, as with most card games. I can imagine some players might stop early because of that. But once you're in the zone, it becomes incredibly satisfying. I'm a big fan of card games, and I think you did a great job designing the core mechanics. Very cool system.
There's one thing I'm not sure about, maybe I'm imagining it. Sometimes I play a card that has both blue and red damage on it, and with the left and right cards it should be enough to defeat the first enemy, but nothing happens. For my current strategy it hasn't been a big issue since I tend to stack a lot of horns, but for players experimenting with other builds, this might feel frustrating.
One suggestion: maybe consider adding a card discard mechanic. Sometimes I just end up with too many cards in hand. Also, increasing the diamond cap could be interesting. It might make ranger builds more viable and shift the balance a bit. Of course, this could already be carefully tuned and I might just not have discovered all the depth yet.
Anyway, fantastic idea. I had a lot of fun with it!