Continue along the cellar until you find the Markza sniper rifle and a Declassify Mission Skull on the nearby wall. Take note that if you picked up the sniper rifle, you'll encounter (dark) wretches ahead and mulchers. If you decide to ignore the sniper rifle, you'll encounter basic soldier locusts and boomers instead.
For this Declassify mission objective, you have to finish the section in 4 minutes before the Hammer of Dawn strikes. You'll encounter a bunch of enemies here so you have to be aggressive and push through the wave of enemies. Cross the bridge and make your way up the ramp. There are some boomers here to wisely take cover and continue pushing forth until you reach the next plaza.
There's a machine gun emplacement there so take it out first then head to the small alley ahead (to the left of the fountain). Follow it and you should spot the COG Skull on the wall. Look on the ground nearby, near the wall to find the COG Tag.
Race against the time and effectively take out more enemies. If you accepted the declassified mission, you have to kill all enemies in the outpost for the door ahead to open. Once enough enemies are taken out, the door will burst open and you'll have to deal with another grinder/boomer and several tickers/wretchers.