Come up to the soldiers that are standing in the elevator (#53). You'll get the Nailgun. Ride down and move straight ahead. When you'll meet Sledge from Rhino Squaw (#54), turn right (the other door are close). You're looking for Voss - he's in small room on the left side.
Commander will order you to find engineer - only him can open the door leading to another part of the complex. Walk out and turn left. Two soldiers will join you, and you'll definitely need help in the close future. In the next room, you'll fight your first battle. Stroggos will jump from balconies (#55), but you should finally deal with them and go forward.
Call for elevator, push the button and ride down. Wait until the door will open, and walk inside the room. Big hatch will be closed, but you can open it with the panel, located nearby (#56). After opening the hatch, eliminate your opponents and repeat this process with second hatch (#57).
You'll see riding truck. The engineer will jump down from the vehicle. Before you'll be able to talk to him, get rid of the Stroggos that will suddenly appear in the tunnel.
Safe and sound, go back with Newberry to the place, where you have meet Sledge. Walk through (closed before) the door and lead engineer through the only available way. Finally, you'll reach the door that Newberry is needed at and he'll fulfill his task (#59).
Shut Down the Defense Grid
You'll be left alone. In the next room use elevator to ride up. Walk straight ahead and jump down, after seeing a broken barrier (#60). Click on the panel (#61) and wait until machinery will uncover second screen. Of course use it. You'll run into single enemy. Eliminate him and go trough the door that he unlocked seconds before.
Regroup with Rhino Squad
There're new orders. To get them, you must find yourself in Nexus building (out of range at the moment). Walk straight ahead and kill all encountered enemies (this time, the biggest challenge will be fighting with extremely tough Gladiator). Finally, you'll get to the main elevator (#62). Use it to ride up. Move through the hatch to finish this level.