Play Last Stand mode with a partner in Co-op mode. Select the Mozdok
Grounds 1 map. Choose three Gadget Upgrades for your suit; Five-Seven
and AK47 (or any gun with a scope) for weapons; and Proxy Mine, Remote
Mine (or whichever required for challenges) for gadgets. Have each
person stand by the door on the second floor stairwells so you are
inside the stairwell. Shoot out the light above you, if one is 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
on the doorway (headshots). If you get overwhelmed, toss a mine at the
door. After you clear a wave, run over to the dead enemies to collect
ammunition for the next round, then hurry back to the stairwells. Repeat
this as many times as desired. If done correctly, you should be able to
complete it in approximately twenty minutes without ever dying.