After the scene, continue upstairs until you meet another human. After he gets splattered, check the toilet room nearby to find a longshot then pick up the Oneshot the splattered guy left behind. Take cover and take out the snipers across the building.
Clear the 19th floor then go down the hole on the checkered floor to reach the lower floor. Clear more grubs inside and make your way to the office-like room. If you operate the lion head on the left, this will open a secret weapons and ammo room. Beside that room is a blocked corridor. You'll find a COG tag marker on a pillar there. Right beside it is a COG tag.
Continue to the rooftop and engage more enemies. After taking out the ground forces, a reaver will land. Take cover and take it out as well. Replenish your ammo then head to the leaning water tower and destroy the two cables holding it down to create a bridge.
Head to the restaurant rooftop and take out more enemies and a reaver. Once done, open the doors leading inside the building.