To shit your pants in battle you will have to consume potions that will raise your mana meter. Once your mana bar is filled all the way, it will say "caution". Once it says that, eat another potion that raises your mana and you will shit your pants. If you need potions that will raise your mana meter, go to City Wok where they sell three seperate items that will raise your mana meter. Unlike the "Irritable Bowels" achievement this can be done on normal battles, not boss battles.