First you have to find Sam, who is in the base marked on the map. Use the car to get there. After a while you will find out that the guerrilla forces are fighting with EDF soldiers Kill all enemy troops.
Sam will lead you to the hill from which you will have a great view on the base entrance.
Leave some explosive charges under the bridge and then (if you have a while) you can block a road with some vehicle
Take control of the turret located on the wall and start the massacre.