Available during the mission The Magnificent Five just after you get control of Hector and are given the tutorial on torches and oil traps.
No need to work out a way to lure 5 guards at once into a trap, just switch to nonlethal and go around knocking out guards. You can use Hector for this since he can knock out the long coats and has fast carry speed. Use showdown with both characters to handle any groups of two.
Form a pile of tied up guards (make sure not to drop them on bushes or the body will disappear) and once the area is clear all you need to do is bring them to the oil trap and dump them in the middle. Go back to the bonfire to grab a torch, light them up and you can add this to the list of cruel and unusual things you've done for achievements.