Contract: Justice for All
The relevant part for this trophy starts after using the wingsuit to fly from one building to another. From this point on, until you enter the security centre, you may not trigger an alarm. Follow the instructions of your teammate. He will tell you how to proceed stealthily. After taking out the MG-gunner you must go up a pipe in the broken part of the building. There you can turn the valve that opens the air vents of the security centre. Turn it a few times, but stop immediately when your teammate says that "targets are coming in on patrol". If you do not stop they will hear you and trigger an alarm! So take them out and return to the valve. Keep using the valve until the air vents are open and toss a gas grenade into the security centre. When the doors open you must take out the captain and go inside to make the trophy pop up. The Ghost Van-Guard can also be very helpful when trying this!