Excellently executed novel idea! The shatter noise is very satisfying, and the enemy spawn rate/locations feels very reasonable. I'm glad that you opted for putting the player's hitbox at the midpoint rather than the endpoints; allowing you to slam enemies is a brilliant way to add variety to the game!
My strategy was essentially to put one of my circles in a corner and focus on the other circle to make very long, sweeping arcs across the map. Dunno if that information is useful to you from a game design perspective, but I had a lot of fun doing it ( :