Got this in Level 6 - "An Ancestral Home." There's a nice farming spot since you can do one terrorize, reload the checkpoint, and repeat until you get this achievement.
Play until you get to the point where you're inside a vent and right outside on your right are a barrel, a dumpster, and another barrel. There's a Hanging point right above the dumpster and a guard patrolling in between the barrels. All you have to worry about aside from the guard is the flashes of lightning in this stage giving your position away if you're not under cover.
Make your way onto the hanging point without detection and dangle a bit (just a little bit) so when the guard passes under you, execute him and his corpse will hang above the dumpster. Soon as this happens, grapple up and wait there - another guard to the right will walk over, see the hanged guard, and become terrorized by the sight of it.
Reload the checkpoint form here and repeat until you pop the achievement.