Sure, here's a simple shooting battle game implemented as a single HTML file using JavaScript:
This game allows the player to control a character (represented by a white square) at the bottom of the screen using left and right arrow keys. The player can shoot bullets (represented by blue squares) using the spacebar key to hit the descending enemies (represented by red squares). The objective is to shoot down as many enemies as possible, and the score is displayed in the title of the browser tab.