I beat the boss, I first tried a character with a sword, and it feel slow, then I tried the one with foam attack/spray, and manage to beat the game, fairly easy. The upgrade don't feel strong, and didn't care too much about them later on because the main attack was strong enough.
At the end, I choose the green gem.... because I like green....
On the technical stuff, I noticed this is Unity which I use too, and I saw some "jittering" with your character movement. That's not normal, it should be smooth, and if not that way, then something is wrong. In my experienced it can be some of the following:
- Using the wrong interpolation method in your Rigidbody component
-Using "transform.position" to move the player instead of using rigidbody's velocity variable.
-Using Update() instead of FixedUpdate()
Also, let the player aim with the mouse.