This is a pretty straight forward achievement.
When you get to the final boss where you have to fight against Princess Kenny, You Will have to fight against Princess Kenny a few rounds. First round normal, then Princess Kenny turns into a Nazi Zombie.
You will have your team mates help you, then they sit out after each round. After a few rounds of fighting against Princess Kenny, You will be prompted by Cartman to fart on Princess Kennys Balls in order to defeat him.
The achievement should pop up after the ending cut scene when everything returns to normal.