Sure! Here's an example of a simple game in a single HTML file where you can fight a boss using fireball powers. The boss will have a shield sliding around the screen, which you need to avoid and hit the boss. Please note that this is a basic implementation and can be expanded upon.
To play the game, you can open the HTML file in a web browser, and the game will start automatically. You can move the player using the arrow keys and press the spacebar to shoot fireballs. The fireballs will collide with the boss if they hit it.
Please note that this is a basic implementation and can be extended with additional features, levels, animations, and sounds to make it a more complete game experience.