This mission is given to you by Solomon Richards ("S" on map) and is available to Michael. When you arrive at the movie studio to start the mission, you will see Rocco and his friend beating up on Solomon. They will run off, so you will have to chase them down. All you have to do to complete the mission is kill both Rocco and his friend.
You can do this by shooting them while they are still in their vehicle, or you can damage their vehicle enough so he stops. Once he exits the vehicle, he will go on foot, so chase him down and kill him. Note that Gianni will be shooting at you this whole time, even in the car, so you should take him out first. When both are dead, drive back to the movie studio to finish the mission.