This really cool and I think has some really good aspects. I like the campfire room with upgrades between arenas, that's implemented really well. There's a few things that are fairly simple to add that would take it to the next level:
-adding collision or avoidance to enemies so they can't stack on top of each other.
-it seems like you can run past the enemies to the upgrade room? You could make the door only open if all are killed, otherwise it makes more sense to just run past them.
-add a simple flashing effect when damage is taken to both player and enemies. It feels better to play, and it just lets you know when you've been hit, or when you've landed a hit. Some kind of knockback or stun would be nice so that they aren't always right on top of you, but that would be slightly harder to do.
The boss battle is cool, love all the different attacks and animations! I wasn't able to beat that penguin unfortunately, though.