Have a fair amount of Life Med so you do not lose your soldier. It also helps if you found body armor. First, find out where one of your reserve comrades is hiding. Run over to their box to lure the soldiers over. While a soldier with an "A" or better in CQC can slam enemies down to the ground and knock them out, this other method is recommended as it does not require high CQC skill, just technique:

Punch a nearby enemy and run behind them while they flinch.
Quickly press Square again to get them in the hostage hold.
Rapidly press Square to choke and fully knock them out (five star K.O.)

Note: They do not need to be directly in front of the box for it to work. There is some leeway in range, but for the last one you may gave to haul them to the box in the hostage hold, then knock them out. Finally, you must use the Codec because after subduing all enemies you are frozen and the mission is complete. To get all of them, make sure the final knocked out enemy's shoulders hit the ground before you press Select.