I found the best place to do this was during Charlie's "Condition Detachment" game. Start
"Updaam" at
"Noon". Make your way over to the left-hand side of the map to find the
"Condition Detachment" island. Cross the bridge, go up the first flight of stairs, continue through the courtyard and enter the main building via the ground floor door.
On the ground floor, you'll find a
large pool of gas on the left side of the room. Inside the gas an Eternalist will slowly emerge, looking to walk out of the gas. Simply fire a bullet into the gas, before she leaves to kill her and earn the achievement.