I went down and fought my way in with two comrad

comrades. In a room on the left hand side, I got the documents. On the right, I took the lift down, and along the way many of the enemy came to their deaths.
Soon you reach the room where the first coast guard gun is. Throw a few grenades in and the enemy will be quiet. Go in and plant the explosives to blow up the guns. There was another one on the left and right, so we blew them up one by one.