Sneak up behind an enemy and target one of their limbs, move forward until you get the backstab icon (Red circle on back), press
to backstab and then press
again to attack the selected limb.
I had an issue where I would do this and the achievement wouldn't unlock, killing the enemy with one hit (or knocking them off the edge) seemed to do the trick. With this in mind there are a lot of low, unarmored enemies in the Detention Centre that are perfect for this.