First I was pretty confused how to play it, then I remembered that you can go out of the bounds.
Then I started hearing the enemies into one big enemy.
I was bit disappointed, that shooting the big guy does not restore some juice or give me extra points, but the idea is kind of the same.
I think it would be great improvement to spawn the enemies inside the playable area, with a brief moment where they can not move or hurt you (ie. Enter the Gungeon), cause now it can be unfair, basically being damaged from off-screen.
Other than that I loved it, it just involves a bit too much luck, that will cause you to lose eventually.
Also loved the visuals, simple but effective. :)