South Park: The Stick of Truth - Pulling Mud Achievement

First, you need to unlock Mana, which you get after Cartman teaches you your first fart spell in Kupa Keep. Then you need to acquire some Mana potions. You can use the potions outside of battle to actually fill your meter. Then go into battle and you'll see your Mana meter flashing with a warning sign. This means one more mana potion will make you shit your pants. Eat the mana potion to shit your pants.
Just remember you can do this achievement at any time after you learn Mana.