1 - destination (tunnel)
Obviously I didn't add a starting area to the map, because you may be standing practically anywhere at the end of the boss battle. There's not much to be done here. Get to the tunnel entrance. It's closed, but one of your colleagues will place an explosive charge on the gate (screen).
Now you must only wait for the explosion to occur. Before you decide to enter the tunnel (screen), it would be a good idea to rearm yourself. You'll find plenty of ammunition in this area, as well as "big guns". Wait for the game to load the next mission.