Head to the garage waypoint first. Once the timer starts, ride your vehicle and drive to the topmost area of the garage. While above, find a vantage point where you can have a clear, overlooking view of the convoy. Once you're set, snipe the dude wearing pink when he goes outside. Jump back into the vehicle, jump off the ramp on the farside area and activate Franklin's special ability to land safely. Now just give the cops a good chase and lose them to complete the mission.
