In Mission 5 you will encounter Envisioned Cultists once you enter the void.
I found that my weapons weren't really doing a whole lot, and getting too close lead to my death as they are very strong. Their projectiles also seem to be able to pull you toward them, so be careful.
I got this achievement by using Foresight to mark an Envisioned Cultist (the first one you encounter after entering the void), then placing a Displace marker at a point I knew it would stop (the golden circles). Once it was in place I displaced into it and that seemed to stun it, allowing me to assassinate it with my sword.
https://streamable.com/rwuen