I think it's one of those games that could have used some more playtesting before being published. I find the logic of the game mechanics a bit scattered.
Some bullets hurt you no matter if you get hit by them or not, others don't. The swipe of your swing indicates that the hit box is bigger than it actually is. You don't really hurt the enemy, but their HP is a timer.
To me it feels like it would have made more sense deflecting bullets/damage towards the enemy, and the song loops for as long as both are alive. That way you wouldn't have to make different gameplay rules for different bullets.
But, all that aside, I think this game has a lot of promise to it. I love that it's so simple and the graphics and world feels really fun. I'd like to see this idea expanded upon and brought further. Especially now when you've got so many players and so much feedback. Clearly, this game is working out for you, so it might be worth pursuing.