At the start of the mission keep your M240 but swap your other weapon for the ACR get your teammate to go outside and come back in again that will make the first wave of enemys spawn. Stay down and don't let the riot shield guys through your perimeter after you kill them all get your teammate to swap his ACR for the riot shield(you'll need the M240)and go upstairs and when the enemies spawn tell them to come back down and do the same. At this bit there will be 2-4 riot shield enemies the best way to kill them is to use a grenade launcher. Next after you've finished them off you should both get a riot shields(swap your ACR's). Now for the next wave plant some claymores about the ground floor and some on the stairs and get your teammate to go up again and repeat the process another 2 times and once all the enemies are dead sprint to the finish.
