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 and get extracted in under 16 minutes for 3 stars. Follow my video guide and you should complete this in about 7 minutes. If you both get killed hit "Retry" to return to a previous checkpoint however this doesn't reset the clock, you'll still be accruing time!
Objective A - Armoury:
I found landing on the highest point of the roof worked best. From here killing the 3 enemies on the roof. Head inside, disable the laser mines, clear hostiles and collect the first nuclear ball. Once collected we get a 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 a few others before dropping down through 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 towards the Comms Centre.
Objective C - Comms Centre:
The material can spawn in one of two buildings. The small building by the river, clear the building and check the warhead on the top floor. If not in here, head over to the larger comms building next-door. There are two possible spawn locations in here, on the ground or top floor. Once you've grabbed the material another checkpoint will trigger. Now we need to run to the exfil site to the north without dying. keep low and use the dirt mounts as cover until the helicopter arrives - then jump in the back!
Do your best not to both die, I suggest taking the medic class, so you can revive each other with the syringe launcher/pistol. If you both die you'll go back to the last checkpoint/collected material, you'll now need to haul ass to catch up on lost time.
I finished the mission with a random guy (no comms) in about 7 minutes in the video below: