We happened to have the jetpack so we simply flew it out to the red spot. When you get close, sneak in to take out the targets. If you just spook them, they'll leap into a helicopter to escape. If you're good though, you can shoot them and the mission is over at this point.

But if you miss, you have to get into the other helicopter and go after them. Note that they'll drag you right over the restricted area so if you don't shoot them down quickly, you will be blasted into tiny bits.

Also note that you can't just leap into the helicopter beforehand - the guys on the ground will blast you to pieces.

He takes the helicopter into town and goes downstairs to get into a car. He goes to a parking garage and you have to follow him there. When you chase him, he waits there for you and you have your final shootout.

mission passed!

Grand Theft Auto San Andreas Walkthrough