Oh wow this is actually a pretty cool game! I enjoy playing it more then I expected! Well done!
The game mechanic reminds me of IO the wisp from dota 2 which I love back in the days hahaha. I can see this being a fun mini game to play on my mobile. Gotta love the powerups you get too, really adds to the progression of the game makes it feel a bit more fresh every new round.
The visual is nice and clean but I feel it'll be even better if the opponents and player aren't just simple shape. One way of doing it is that you could add a theme, for example you could theme it around immune system. Perhaps you start as a single cell that expels antibody to defend itself and those enemies are viruses/bacteria coming to attack you. And maybe every upgrade you'll add a new cell making you more powerful. Anyways that's just one of the many possibilities on how you could take it further ^_^ or at least something of sorts.
Oh also I think you should add a indicator where to bullets are being shot from, it'll allow the player to position and plan their rotation better, especially when the enemies are getting close.
Overall I really like this game, well done! keep up the good work!