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.
This achievement does not need to be done all 3 times in a single battle; it is cumulative. I recommend making a save and then doing it three times in a battle back-to-back to get the achievement, then reverting saves to avoid wasting the mana potions.