Kill It With Fire!

Kill it With Fire! is a dungeon & raids achievement earned for defeating 3 Burning Souls during the High Prophet Barim encounter in Lost City of the Tol'vir on Heroic difficulty.

Strategy
There is no mob named Burning Souls, instead you have to kite the Soul fragments, spawned during the "Twilight Phase", over a patch of fire so they get a debuff and kill them while they have it.

Trivia
The title of the achievement could be a reference to a Dragon Ball Z Abridged episode where Bulma exclaims "Kill it with fire!" after seeing Zarbon transform.