I reached level 10 but couldn't defeat the boss due to the sheer number of enemies. I thought once the boss spawned the enemies would stop spawning, but they didn't, so I was eventually overrun. An effect to increase damage range could help, or some upgrade on bullet speed. Being able to autoshoot would have being nice too, since you're already auto aiming and that timing your shots isn't necessary in this game. Spamming the z key hurt my hand after a while.
But that being said, this is a really well put together game, it's a lot of fun to play! It's one of my favorites of all the games I played. Great work!