REWARDS - Respect, $18,000
Get into a car and head towards the yellow blip. As soon as you get there, Toreno's men start firing at you. Get out of the car and fire back, then head into the alley and go up the stairs to the heli-pad. As you get towards the top, the chopper will lift off. Head to the top, finish off any guards and grab the marked rocket launcher. If you're having trouble with this part, you could snipe the guards from a distance.
Get a nice and fast vehicle for this next part. The bike in front will work. Head south on the street in front of the pad, following it as it curves right, then take the first left. Veer to the left and then right to get onto the freeway. Luckily, the chopper will follow the freeway so you don't have to take any other roads.
Get a good distance ahead of the chopper and then get off of the vehicle and switch to the rocket launcher. Get off a few shots when the chopper approaches. If you miss, get on the bike again and get ahead of the chopper once again. Keep doing this until you hit the chopper once with a rocket and send it crashing into the ground.