This achievement can be a little bit tricky to get because you have to slice instead of just simply killing them. You can go into focus mode by press and holding right before they attack you. Press and hold down until the light on your blade reaches the tip. Release it right while their attack is coming at you and slice them in half to unlock the achievement. Another way to do this is during the chapter “Prelude”, go to the area where you have to fight a lot of ronin’s near the end of the level. Enter over focus mode by pressing and wait for an enemy to try and attack you. Just attack them once to cut them in half, doesn’t take as much time as a perfect slice and you only need one attack as opposed to multiple light and heavy attacks.