Nightmare Master Achievement
Complete chapter 7 in Nightmare Mode.
While on Nightmare mode you need to complete the last chapter prior to the credits. Unlike the standard story mode, you will be timed, so you must focus more on completing story missions and getting the items most important for finishing the game.
You can either do Nightmare Mode first, or wait until you complete Story Mode and turn Nightmare Mode into basically a New Game +.
Note: You can also get this by joining a friends game if they are in Nightmare Mode chapter seven and completing it with them without doing the first 6 Chapters yourself.