When I read the description, I thought this achievement was about killing those enemies that look like statues. But the ones you need to kill are the house leaders (you know, just like in the main game when we kill house leaders, fortress leaders, camp leaders etc)
I just unlocked it after killing the leader in the Fortress of the Three Sisters.
Please, if you have a better solution, feel free to add your comment to this thread. ;)
Hope it helps and good hunting!