Really cool idea and beautiful art! I liked it a lot. It was a shame you could not quite finish the enemies, but I think it does not subtract from the overall experience :)
I also have a bit of criticism, but dont take it too seriously, because I completely understand you did not have time to finetune everything in only three hours! :D First of all, it felt a bit inconsistent how high my knockback with the gun was. The one time I tried to do a super jump I got really high, whereas other times it really gave just a little push. Is there some mechanic I am missing here? Secondly, there is not a lot of space at the bottom of the screen to click if you want to do a bullet jump. Especially when you are already doing badly, it is only getting harder because it becomes more difficult to do the super jumps again. Maybe you could make the 'death line' be a bit higher than the bottom of the screen, such that there always is some space? But maybe this is just my frustration at being bad at the game speaking :P Last of all there was a little bug where I could get stuck in walls, but that is just a minor thing.
Of course some music and sounds would have been amazing, but I am sure you know this and simply did not have time :) All-in-all a really cool and creative concept, which was executed really well too! The little screenshake when you shoot felt great, and the massive bullets really helped with clarity and the feeling that it is a huge gun you are holding. Great job! Hopefully I was not too critical ^_^