I cheated to finished this mission, but you should be able to complete it rather easily if you have the following two items, a tank and a sniper rifle. *Leave your tank at the damn before you start the mission (this will save you a lot of driving time). *Once you escape from the Mansion, get your sniper rifle and go pick up your tank. *Take out all the people on the ground with your tank then go to a safe distance to snipe all the people that are higher up. *Be sure to pick up one of the M16's from one of the dead bodies before it disappears too since you need this item (or a rocket launcher) to take out the helicopter.