Well, this game's pretty fuckin kawaii <3
I'd say having a skippable demo that appears before you start the game would be ideal, there's a lot of arcade games that do it that way, because otherwise figuring out how to use the balls as projectiles is pretty unintuitive.
I'd also suggest completely getting rid of the stamina system, because it forces you to look at a bar away from the action in the screen. I think you could get away with the melee attack not even doing damage to enemies at all, or something ridiculously low like 20 shots to get one enemy (might be good for score farmers).
This game has a lot of potential, you should try experimenting with different types of weapons as powerups.