Move the Mareep using the arrow keys or WAD. You can also use Space to jump. Avoid touching the other Pokémon or their attacks. Go to the purple/pink arrows to get to the next level. If you eat a berry, you will become invincible. You will then be able to defeat Pokémon by touching them. In the last level, you will have to defeat the boss using a berry like this.
PREPARE TO RAGE!
