There is an easier way but I cannot edit the page: If you have collected 80 packages, use the Sea Sparrow parked at Diaz's mansion. Fly it to the cars blocking the entrance, park it a safe distance away and use the guns to blow up the cars. Then move the sparrow up to the gate and get out. From here, arm your favorite weapon and move your way to Lance. Once you rescue him, work your way back to the sparrow. You will be in the air and half way to the hospital before the comets arrive. If you do not have 80 packages, use the VCN helicopter because it is the fastest. Use granades, rocket launcher or the mini gun on the cars - everything else is as above.
Another alternative; blow up the cars, then fly the sparrow to the building with Lance. Park it quickly and get out before it catches fire. There is usually a Sentinal right outside the building and often a garbage truck nearby, so you have two options, speed or power. With the Sentinal, as you are driving away, start on the left side of the road and then turn hard right just as the comets are nearing you. They will all swerve by and ram into each other and you will be off to the races. With the garbage truck, just run them over.