In the fourth memory, "The Lady Talks," Evie will go to Owens manner. Once you arrive the game will give you a quick tutorial on a Brute's role in the fear system. Brutes are immune to fear, and when they see panicked enemies they will calm them down. The key to this trophy is finding a group of enemies near a Brute. You want to prevent enemies running in all directions towards different Brutes, making a perfectly-timed kill nearly impossible.
The way I received this trophy was to perch myself on one of the banisters at Owens Manor near the vantage point. Below you will see a group of enemies and a Brute. Toss a couple of fear bombs into the group and wait for the Brute to begin maintaining order. As this happens air-assassinate the Brute and the trophy should pop.
Good luck!