South Park: The Stick of Truth - Dog Whistle Achievement

Achievement - Dog Whistle
Fart on a canine enemy while in Gnome form.

After you progress in the story, you'll unlock the ability to shrink to Gnome size. Now you must fart on a canine enemy. This can be done either before battle if you Cup A Fart, or after you've defeated them, then you can fart on their body.