You must have the "Hammer of Infinite Fate" DLC for this achievement. The "Eye of Sorrow" can be purchased as a Primary Weapon for the Deuce in the Motorforge for free. After equipping it, circle around the Jungle area and kill all of the Drowning Doom units you see with the Eye of Sorrow until the achievement unlocks. Location of units may differ if the game has not yet been completed.
Posted by Runch Optimus: "Just a note: Kills must be made during open world sections. Any kills during side missions or stage battles don't count towards the achievement."