On Vegas Junkyard, when the mission starts go to the ladder by the right truck.

Go down the ladder and walk until you come to another ladder.

Tell your team-mates to go to the corner that you came from to kill any terrorists following you.

Fire one shot off. Terrorists will begin climbing down.

Kill those fools for as long as you like (realistic difficulty is recommended).

If you need help send me a message and I will be glad to assist you (Blak Comet).