I really liked this game and the use of the theme in it. The concept can definetely be expanded upon with some polish and extra levels. I liked the resource gathering during the level, it gave me a reason to explore, although I did feel like 3 minutes was a bit short and I never actyually got to the coolest upgrades.

I liked that every room of the stage was designed differently and had a unique problem to solve to get the resources.

The boss itself is really cool, and I like that it uses the attacks that you learn during the stage (like the shockwave and slime enemies). The music during the fight is also very nice. 

Also, I feel like the game is rather hard and unforgiving, especially because you respawn all the way back at the beginning of a stage. I think hearts should be able to be crafted with a seperate resource, because every time I healed, I didn't have enough resources to upgrade the gun.

All in all, I think you can see this game as a big win, very cool!