Here's a simple implementation of the game as a single HTML file using JavaScript and pictograms:
Zombie Game
Score: 0
In this game, you play as a blue character represented by a blue rectangle on a canvas. Zombies appear on the left side represented by a yellow rectangle. To kill a zombie, tap or click on the canvas, which will trigger the `shootZombie()` function. If you kill every 20th zombie, the zombie will be killed and your score will increase. The score is displayed at the top. There's also a very low chance (one in a million chance) for a green zombie to appear.
Please note that this is a basic implementation and can be expanded upon to add more features and complexity.