You must beat this mission on Professional for the achievement. The mission is called "Miami Alternate Ending" and is not terribly hard if done with a partner in online co-op. Mission 8 is not terribly hard until the end where you face off against Dalton. To defeat him make sure your both packing stingers. It should take about 4 rockets to knock him off the catwalk as long as each one was locked on. To easily pull this off make sure once the cutscene ends you run out side of the vault and shoot the stingers from outside the vault door, Dalton will keep circling around so take your time and wait until he walks into your line of sight. Once he is knocked down a cutscene will start. After it ends run imeediatley to one of the little bunkers, they are where all those electrical units are and they are lower down and provide great cover. WATCH OUT for a guy on the second floor who will be firing rockets at you, try to take him out quickly. As for Dalton turn on your GPS to see where he is, try chucking grenades at him and shoot him when he occasionally pops out of cover, try to keep moving around though from "bunker" to "bunker". What my friend and I did was eventually he got into 2X Damage mode and I was in Stealth mode, we were both close to Dalton so he started firing on Dalton with 2X Damage and I ran over in Stealth and started firing into his head. I suggest using the gun with the most damage which is the PKM, as your primary weapon. Anyways once your stealth/2X damage mode is done you should have Dalton down to literally no health if not dead, if this is the case just keep shooting him once the mode ends, and he should die. He died for my friend and I literally once my stealth mode was done.