Overall nice game. I really like shooters and the simplistic graphics.
When I got to the boss level I didn't know how to beat him so I was just shooting enemies
he was spawning. I saw the hp bar but didn't see it drop.
Just a tip for the music, the beat was nice but you should put more different instruments into
the mix. As a tip from me (not a musician at all) you could work with a bit slower beats per minute (bpm) I think you used the basic (for bosca ceoil which I also use :D ) 120bpm. I suggest going
lower than 90 and picking one of these (40 - slow could be used for calm games
, 80 - decent for most games, 90 to 100 agressive requires lots of work )
and also make the song longer even if it's just a loop. Unity has this problem that it loops
the song not smoothly so you can hear the cut out sound at the end. To make it
not so noticeable make the songs longer.
Overall nice work!