This one is really easy to get in memory sequence 4 of the DLC. You will come to a spot above 3 regular enemies and a brute.
Throw 2 'fear bombs' to panick the enemies (the symbol above their heads needs to turn RED). Quickly switch to your throwing knives and aim for the brute's head. Kill him once he starts to calm down the panicked enemies.
Here is a video of me unlocking the achievement:
This is also a great spot to grind out:
Just throw a bomb, scare the enemies, let the brute calm them down, scare them again and restart the checkpoint once you are out of ammo.