This is completed in the co-op mission Exfiltration
This can be completed on any difficulty so for the purpose of the achievement run through it on Easy.
Thanks to blackstockc for being my wingman.
Teamwork and communication is crucial, you need to execute targets at the same time or alarms are raised instantly and you'll need to restart. That said the achievement doesn't require you to complete the entire mission, only make it to the VIP room which is fairly close to the start.
First room will have two enemies in front of a tv, coordinate a countdown to fire together and take them out. Progress through the room, down the hall and stairs to your next two targets and same as before execute together, headshots work best.
Creep around the corner slowly and you'll run into your first CCTV camera through the doorway, a well placed bullet will dispose of the issue. The next two corners will have CCTV cameras as well.
Progress silently through the hallway and you'll encounter an enemy in a doorway, sneak up and knife him to put him down silently.
These next two encounters are the only ones that are sort of tricky. Creep up the stairs and have one player execute the guard while the other player simultaneously takes out the camera.
Up the next flight there will be two guards and a camera. One player targets the camera while the other lines up the guards head in the background. Simultaneously take them out and IMMEDIATELY converge fire together on the guard in the foreground. To help your chances when lining up your shot on the background enemy try for a through and through shot from foreground's torso to background's head.
** A couple people have mentioned in the comments they're encountering two enemies in the hall outside the VIP room after the stairwell which need to be handled covertly as well. I personally didn't have these additional two in my runs but BE AWARE they could spawn for you and need to eliminated silently before proceeding.
If you've managed it to here without setting any alarms off the achievement is yours in the next hallway when you approach the VIP room. Have fun going loud from here on, the two enemies inside the room are of no consequence to the achievement.