Sure! Here's an example of a simple 2D side-scrolling beat 'em up game implemented as a single HTML file using JavaScript:
In this game, you control a player character represented by a red rectangle, and you can move it left and right using the arrow keys on your smartphone. There are enemy characters represented by black rectangles that move towards you from the right side of the screen. The objective is to avoid colliding with the enemies for as long as possible.
Please note that the game logic and visuals provided here are very basic and can be expanded upon to create a more complete and engaging beat 'em up game.