I really love this one. My favorite so far. The idea to use the countdown as an attack window, and to chain it with special abilities is genius, and I think by adding a bit more depth (Special ammo types depending on the number you set? Pick ups that prolong the timer? Ability that work when you are setting up the countdown?), you would have material for a bigger game.
I also loved the visuals, they were pretty cute, and the story was funny.