This game was really good for a first game. The only suggestions I have is to give the boss a health bar so the player can feel progress, and maybe make the bigger circle guys have either less health or make their movement less random. Maybe they can bounce off the walls instead of also changing movement while in the middle of the screen. I however have never made a game before, so take my ideas with a grain of salt.
Edit: Whenever they get to the center of the screen, they bounce like they hit a wall.