Declassify Mission: Frag grenades will be planted around the Beach landing with reduced access to the locust defenses.
You can start the declassify mission immediately by examining the skull ahead. You have to move from cover to cover but you need to wary of the planted frag grenades. You can shoot them to detonate them early. There's a Breechshot in the right side of the Beach, which you can use to snipe the gunners in the bunkers ahead.
Don't clump yourselves together; if you see your teammates moving forth, move on your own pace and direction. Proceeding close together will just increase the risk of getting hit by the mortar shower. Destroy the barricades as you proceed. Once the bunkers are cleared, you can move in and engage the foot soldiers.
More enemies will be waiting behind the bunkers so take them out as you see fit. Once cleared, you're free to move to the next section. Before doing so, you'll find a COG skull near the exit so go past it and enter the room with the Onyx ammo box. Beside it is the COG tag.