When you open the doors in the back of the courtyard, you will be ushered into the main atrium of the City Hall building. Now, you will just have to fight your way through the area and destroy the evidence. Unfortunately, a team of Spec Ops soldiers will block your path. Sprint towards the bench directly in front of the door and take cover behind it. Fire out from over the bench at the Spec Ops squad, who will be taking cover in the center and sides of the room. Be sure to keep an eye on your right flank, as more soldiers will mill about over there and may attempt to flank you. If you are playing with other human players, have half of your squad cover the right flank while the other half shoots straight into the room. If you need to, use grenades or Stun Grenades to get the enemies out of their cover.
After you defeat the first group of enemies, don’t leave cover. A second wave of enemies will come rushing out from the right side of the level. Transition from one side of your cover to the other, so that you are protected from your right flank, and fire over cover at them. If your position gets overwhelmed, head towards the doorways and take cover there. Alternately, right after you beat the first wave of enemies, you can sprint towards the right-hand door where the enemies will come filtering through to get the jump on them. Hit them at close range with a Shotgun as they exit the room to blow them to bits. Just be sure to keep an eye on your health, as it can drain fast in close quarters. Use melee attacks to take out any stragglers.
After you kill the second wave, a third wave will come in. This wave will begin from behind where you now stand, towards the left side of the room. Again, run around the cover and reposition yourself so that you are facing the left side of the area. Enemies will begin to stream in from the doors. If possible, set up a pincer movement, having some of your squad members take cover towards the front of the room, some of them towards the back, and you at the center. Catch this wave in your crossfire as they exit the doors to mow them down quickly.