Select Co-op Last Stand mode with a partner. Choose the Mozdok Grounds 1 map. For your suit, select 3 Gadget Upgrades; weapons are Five-Seven, AK47 (or any gun with a scope); and the gadgets as Proxy Mine, Remote Mine (or whichever required for challenges). Have each person post up at the door on the second floor stairwells, so that you are inside the stairwell. Shoot out the light above you (if present). Look directly out the door or to the right (depending on which stairwell you are in) to see a doorway. As long as you stay in the stairwells it will force the enemies to only spawn from that doorway. Just focus fire the doorway (headshots). If you get overwhelmed you can toss a mine at the door. After you clear the wave, run over to the corpses to collect ammo for the next round, then hurry back to the stairwells. Repeat this as many times as desired. If done correctly, you and your partner should never go down and it should only take twenty minutes.