Really great attempt. Impressive for a first game.
The second level seems out of place. Seems like it should be towards the end because of how difficult it is.
I got to the boss but gave up, purely because there's no health bar. I had no idea I was going to be battling for. Lack of health bars in the biggest issue. Second, it really needs auto fire option, or at lest fire on the mouse being held down.
Another big reason for giving up was the boss' movement. It moves faster than the player, and it's random. I had a few attempts where it seemed to follow me and it was impossible to escape. It was pretty much over me the entire time. A pattern with tells would be fairer.