First select single player terrorist hunt.

Set the map to Vegas Junkyard and the difficulty to realistic, and the density to low.

When the game starts hide behind the truck/lorry, and fire a few shots.

The terrorists will come for you so just pick them off until there are only 12 left.
Then move out from cover and go to the ladder near the furthermost lorry on the left.

Go down there, and walk through the tunnel until you reach the ladder.

Stand really close to the ladder and al the terrorist will come down one by one.

Easy CQB XP.