Conan

I did this during the serpent Queen Boss battle by using the Fire Rain Armor ability when she creates a circle of enemies around you.

For this achievement, it asks you to kill five or more enemies at the same time.

I earned this during my trip through the Dark Prison for the first time. If you are a near pool of oil and oilmen are being created, make sure there are five of them. Once they are near you, in a circle of sorts, now's the time to unleash a Rain Of Fire. Hopefully, it will only take one shot and it will kill them all. If not, just reload your game and make it back to that spot and try again until you do. It also may take some running around to get them to follow you and to stay within range of each other before performing Rain Of Fire again.

Have fun and good luck!