That was pretty fun! The hearts dropped made it so it never felt like you run was doomed as there was always a chance for some health to appear. I didn't manage to beat it the first time since I didn't find the gatling gun on my first run and the boss hit me with a death lazer. But second try with the gatling gun I managed to beat it! The game was great and the couple levels in there gave a good idea of what it would look like expanded, it could also make a really fun endless game if you made some procedural spawning mechanics!