Sure, here's an example of how the game could be implemented as a single HTML file with JavaScript:
In this example, the game consists of a game container (`game-container`) in which the player (`player`) controlled with arrow keys can move left/right and jump. The game also includes three blue platforms (`platform1`, `platform2`, `platform3`) that the player can jump on. The boss (`boss`) appears at the right side of the container and shoots blue projectiles (`projectile`) at the player. The boss has 5 health points represented by `bossHealth` variable.
You can copy and paste this code into an HTML file and open it in a web browser to play the game.