The game has a very good UX feedback, every shoot feels so good, especially the combo, it is just SOOO satisfying, Good job!
Here is what I was frustrated at while playing the game. The difficulty ramps up very fast even in a single level. It can be very difficult once there are so many tracks of balls coming without pause. Particularly, when it gets halfway near the damage zone, I felt "helpless" when I wanted to hit the ball closest to the damage zone, but I was instead blocked by the other train of balls since the player is only shooting from the bottom of the screen (while the damage zone is somewhere in the middle).
Also, I find it punishing that when you killed a goblin, it drops some knives? (swords?) drop and if it hit your character you take damage. I was expecting to be rewarded but felt punished instead.
I stopped playing after succeeding 2 and failing 2 levels.
For the upgrades/buying stuffs between each level, I just don't really want to bother with it and move on to the next level.
Overall, the game has very good moment-to-moment feeling. I feel it can be better if you reduce the difficulty just a bit (or is it my personal skill issue? LoL). I would suggest also giving each hero a special ability (with cooldown between uses) since it is in a medieval fantasy setting, just to help with overwhelming tracks of balls.
Great work!