This is pretty straight forward. You will either need a facility or a friend who has one for the doomsday heist to be initiated. This is for completing all three acts of the doomsday heist. once the cut scene after completion is finished the achievement will pop along with the achievement “The Doomsday Scenario” if you have not already unlocked it. Difficulty does not matter nor does act completion continuity I would recommend doing them in order for the story aspect of it.