I used Timbol Time trial for this. You must defeat a boss before you can do their time trial. Time trial is accessed from the main menu. When you choose timbol, a infinite amount of cars will spawn, just jack one, then get out after the door closes, then run away as the gang will shoot you. DONT SHOOT BACK. if you do the gang members in cars will get out before you can jack them. The members in the car you jacked as well as any gang cars near you, will get out to hurt you, but if your run down the street, they will ignore you, and you should run into another car as your running away from the first one, letting you gain another jack count.
Or, there is a quicker way, which is detailed in the video below. Its the Diaz Time Trial from the Los Muertos boss's. When you run out of cars to jack, kill yourself or kill diaz, and start over. Eventually youll get the achievement.