Once the mission starts, you'll have to choose whether to drive or shoot.
This is an easier choice out of the two since you just need to drive and steer clear of obstacles. Your partner will take care of the targets for you.
As the gunner, you have to take out the main targets to clear the path. Don't mind the small fries; just keep shooting at the main, marked targets and keep good intervals between shootings to prevent your MG from overheating. Take out the enemy vehicles, RPG troopers and other marked targets until you reach the compound. Watch the scenes and the section will be complete.