This Achievement is for killing a demon with Kaede's special attack. This can be obtained in either of the two instances you control Kaede.
Find a single demon and press your stance button "RB, R1" and push forward to perform a dash (direction depending on character position) as Kaede's animation for leaping over the enemy is happening hold down your attack button once you land if you are directly behind the enemy then she will grab them by the neck and cut it this will one shot the enemy.
**Video Guide, Starts at 00:38**