A cut scene will begin of Kim giving orders to Marcus and Dom. Walk up to door and kick it open.
Make your way across this big open area to the building in the background.
When Marcus reaches the foot of the stairs, Locust come out of the building. One will be manning a Troika gun emplacement. Flank to the right side of building where the grenadier is. A Grub hole will open in the building. Blast the Locust that come of it. Proceed to the gun emplacement. When all Locust are dead, a cut scene will begin.
After the cut scene Locust will rush up the steps to where you are. Use the Troika and take out the Locust.
When they're dead some Locust will be in part of the building to the right. Enter that part of the building and shoot them. After they're dead Kim will open door. Enter the next room through the now open door.