In the territory Ka-Khem Nome
is a outpost called Dasos Hideout
nearby a fast travel tower. In this camp are sleeping four enemies in a tent at nighttime.
Switch to nighttime and go to this outpost. There is a fifth enemy in this camp, so kill this enemy first silently. To the left of the tent stand a small wooden tower with 3 oil jars.
Put this oil jars in the middle of tent silently. On the right side of the tent you can light up your arrow. Shoot with this arrow to the oil jars and the four enemies are burn to death.
Now kill yourself with fire and repeat this process until the achievment unlocks. Note: The skill "Pyromaniac" increase your fire damage by 50% so the enemies die more quickly. (not 100% sure)
Note from the comments: You can also knockout the guards or use sleep darts on the enemies before you light up the oil jars.