The following cheese has been patched: Rumor has it you can run through the mission loud and collect all three objectives. Once you have the third and final objective - get both yourselves killed and "Retry". This should reload a previous checkpoint and pop the "Going Dark" achievement. A full "legitimate" walkthrough can be found below:Done on Online Co-Op Mission: Low Profile. You'll need a friend to help you with this as it can't be done solo (unless they disconnect) nor via split screen. The objective here is to collect all three balls of nuclear material without raising the alarm. You are OK to kill hostiles as long as they don't fire/shout and raise an audible alarm. If they do, simply get killed and "Retry" to return to a previous checkpoint. There's a total of 4 checkpoints within the level.
Objective A - Armoury:I found landing on the highest point of the roof worked best. From here killing the 3 enemies on the roof, which in turn will alert 5 more from within the building itself. Wait on the staircase and shoot each one as they come through the door. Now with most of the building clear we can head in, disable the laser mines and collect the first nuclear ball. Once collected we get a new checkpoint. Head back up to the roof and parachute off towards the Barracks.
Objective B - Barracks:Climb up the ladder on the back of the building to gain access to the skylight. Directly under the skylight we'll find an enemy, shoot him and camp out his body until you've removed everyone from the top floor. When quiet drop down the skylight to collect the second nuclear ball. Here another checkpoint will kick in - do your best to exit the building from the ground floor and head west. If you get spotted just die and "Retry" to respawn again under the skylight.
Objective C - Comms Centre:This one I found the trickiest of the three! The material can spawn in one of two buildings. For whatever reason I couldn't get into the smaller building without raising the alarm. So would just die and retry until it appeared in the bigger building. In the larger comms building you can actually get in via the balcony by mantling on top of the blue cargo container, climbing up and walking through the door. Here you'll need to clear one guard to hopefully collect the last objective.
Once you pick up the third objective the achievement will pop. You don't need to do the escape section quietly!
Check my fully narrated video for a full guide: