Declassify Mission: Finish before artillery strikes in 4:00.
You'll find the declassify mission skull ahead. If you accepted it, you have to move and finish this section as quickly as possible. Continue forth and take out the enemies. There's a silverback suit in the middle so you can just run towards it and equip it.
The silverback's main weapon is the mounted machine gun. It has infinite ammo and will just overheat when used over time. You have to press RB to force it to cooldown. Press A to make it run, then press A while turning to make it turn fast. Press and hold A to deploy it to the ground so you can fire four rockets. The rockets are very powerful but very limited as well.
As you get past the gate, take out more enemy reinforcements. There's a COG tag in the barracks to the right. Just follow the trail of blood leading to a door to find the COG tag in front of it.