P-Minus

P-Minus is a 2D top-down shooter where enemies are coming. They've got numbers. The player has a gun. Shoot enemies down to zero before they reach the player. But watch out—some of them merge, making the numbers bigger. And some of them explode when they die.  Survive as long as possible. It only gets harder. 

Theme implementation:Enemies have numbers. Those numbers count down to zero through player action. The player IS the countdown mechanism. Every bullet is a tick downward. Every hit brings the enemy closer to zero.