Collect weapons, and get all four survivors into the departure room with the radio (AI players need to be herded into the room). When ready, use the radio to call a helicopter.
The finale is a long-range trek from the safe room to the helicopter, with hordes spawning at regular intervals.
Check the building for the left if you don't have enough supplies (usually if the AI players don't pick up medikits or weapons).
As you go through the canyon, there are two drops. The first returns you to the river. The second is identified by a waterfall, where you can get some medikits before the enemy tank attacks. The river is blocked after the tank. To proceed, climb up the far canyon wall (marked by flares) and reach the ammo pile. Continue along the river until you reach the storm drain.
When you reach the storm drain and climb up the right path to the ammo pile, the final horde commences. Run to the helicopter; if you are going left of the wreckage, you will also need to move around a fallen tree as well. Climb the ladder to escape.