Take a car from Vercetti Estate and drive to the point on your map. First, take out the two guards. The easiest way to do so is to run them over, then get out and talk to the bar owner. After that, he will have told you the location of the security. Go over there and kill them all. Start off by blowing up a car to kill as many as you can, then kill the remainder, if there are any. After finishing off the rest, two more will escape on two PCJ-600s. Chase them down in a car and run them over (you can use a firearm, but running them over is the easiest way). Then you have completed the mission.