Your first encounter with the freaks will be during "L'Opera Dramatique
." Freaks are the enemies that carry a silver hook in each hand. In order to impale a freak, you must beat them down until they are sparking (a button prompt appears over their head). When this happens, walk up to them and hit the
button. The Dishwashwer will impale them on their own weapon. You'll need to do this 13 times. Now, there are not enough of them to earn the achievement on this level, but there will be plenty more of them in the following levels to do this. So, I'd suggest just playing through the levels as you normally would. Eventually, the achievement will unlock.