Sure! Here's a simple implementation of a 2D shooter game with red enemies using JavaScript in a single HTML file as per your requirements:
In this game, you control a blue player character using your mouse. The objective is to shoot the red enemies (represented by red circles) with green bullets (represented by green circles). Each hit on an enemy increases your score by 10. The game runs indefinitely until you close the browser tab.
Feel free to modify and enhance the game as per your requirements or add more features to make it more enjoyable.