Sure, here's an example of a shooter game implemented as a single HTML file using JavaScript:
This game sets up a game area with a player element (represented by a red square) and spawns bot elements (also represented by red squares) every 4 seconds. The player can move horizontally by dragging on the game area. By touching the game area, the player can shoot the bot, earning kill points and potentially leveling up their gun. The spawn interval decreases over time to increase difficulty.