After few steps you'll see a declassified mission symbol on the wall on the left. When you activate it, you'll have 3:30 minute to clear this stage and reach the end point. After activating it quickly jump over obstacles and go through the short tunnel. You can quickly run to a distance Silverbak and kill all nearby enemies with it - this will shorten the fight but you'll be more vulnerable to enemy fire. You can also stay where you are and fight from behind covers. The important thing is that you need at least 1:45 minute before entering the next area.
After clearing this are, the gate in front of you will be opened. Position yourself in front of the gate, watching out for enemies on the right and place shields, thus enabling missiles. Use them to get rid of enemies hiding behind covers and larger units. Use also the car wreck on the other side of the gate, as cover for yourself.
After clearing the other side of the gate, go to the building on the right. When you get closer to the end point, you'll find a COG tag near sandbags.