I use BLAZE VII's method. All credits goes to him.
This challenge will finish on 30th October.
I rewrite its guide:
This is how I got it done fast:
1. Load up "Ground Zeroes" (Normal difficulty).
2. Tranquilize the guard in the watch-tower.
3. Tranquilize the two guards to the left of it.
4. Wait for the guard in the jeep to arrive (coming from the right). Tranquilize him too.
5. Carry all guards to the top of the hill (where you started the mission).
6. Run down to the fence entrance to trigger the checkpoint.
7. Now call in the chopper.
8. Put all the guards in the chopper. As soon as the last one is extracted. Reload checkpoint!
9. Rinse and repeat!
Note: Sometimes one guard will disappear because there are too many objects at once on top of the hill (unlocked weapons etc.). That's why I wouldn't recommend to get more than 4 guards up there.
Also it counts for the challenge as soon as you put a guard into the chopper, no need to wait. You can check your progress immediatly in the achievements/challenge section!