Run upstairs, and after that go into the corridor. You'll be attacked by a few Drones. Hide yourself behind the first barrier and shoot. They shouldn't make any problems. After the fight, your teammates will show you a place on the ground, where 4.Tag lies.
Take all the ammo and grenades and go upstairs. After a while you'll be attacked by monsters. You have two choices here. You can simply shoot at them and kill them all or you can run forward and throw a grenade in the hole (to close it).
Later you'll be attacked by the next group of Locusts (what a surprise, heh?). There isn't much to do here - your team will do most of the work for you. After that, follow them, pass the bridge and enter the building.
Just after that a hole appears and with it Locusts of course. It's easy to close it - get there (using the left side of the room) and throw a grenade in. After taking a few steps another hole opens. Run forward and use a grenade, then step back. Find a good hiding place and shoot your enemies to your heart's content... or till they're dead. Before you leave this building, take 5.Tag off the grass. Now go towards that corpse you've seen while on the bridge.