When fighting the bard in the sleeping donkey, he will play the "brown note" and it will target you and make your mana overload, after it overloads, you will shit your pants thus giving you the achievement. You can also get the "Pulling Mud" achievement in this fight as well. Another way to do this is to eat mana potions untill your mana bar is striped out with a caution symbol next to it and you should shit your pants the beginning of your turn but just remember unlike the "Pulling Mud" achievement this HAS to be done in a BOSS FIGHT as Pulling Mud does not