MGSVPP: Farming Staff

Yeah! There's actually an excellent way to farm staff in MGS:TPP. Replay mission 16: Traitor's Caravan! In this mission you have to find a truck, defeat the spawned Skulls, and fulton the truck back to Mother Base. The truck always spawns in the same spot in the back of the airfield so it is easy to perform this mission, and thus the farming, quickly.

Partway through the mission you'll need to fight four Skull agents. After the Skulls have been slain all guards at the airfield and the several surround outposts will be knocked asleep for several minutes by the Skull's powers. Using D-Dog these sleeping soldiers can be identified and easily and safely fultoned back to Mother Base. Once soldiers start to wake from their slumber you can return to the airfield to easily extract the truck and finish the mission

When doing this you should fulton ALL soldiers due to a quirk of how soldiers' ranks are generated. The more staff you have at Mother Base the stronger the staff you find out in the field. It seems counterintuitive but only collecting high-ranking soldiers will mean that the rest won't be as strong. Every D-level soldier you fulton back slightly increases the odds of finding an A-rank later. This means it isn't in your best interest to dismiss weak soldiers either; instead keep them in staff roster even if they aren't assigned to a facility.