I found that the easiest to do this Trophy was at an Outpost called Dasos. In this camp are four sleeping enemies in a tent at nighttime. Switch to nighttime and kill the fifth enemy silently who is usually standing around the wooden tower. Once you have dealt with him, pick up the three oil jars and put them in the tent with the sleeping soldiers. Light your arrow on fire and shoot at the oil jars. This should kill all of the enemies in the tent. Kill yourself and repeat till you have unlocked this trophy.
Note: I did this without the "Pyromaniac" ability unlocked. Not sure if it helps.