Easiest way to do this: Get the Bear (the armored personnel carrier). It's pretty close to invincible, especially if you upgrade it, and it has its own gun. You never even have to get out of the car. It's especially great on the Airport run; don't have to worry about the Brotherhood constantly destroying your vehicle and having to get another one.
Note that you don't have to destroy the vehicles coming after you; just kill the driver and they'll quit following.
If you haven't got the Bear from completing the Ultor mission, just get about 5 stars (4 in Saint's Row) and the police will start setting up roadblocks with Bears. Hijack one quickly and run to a Forgive and Forget. Then save it in your garage, and get all the upgrades. It's a great vehicle (also great in Snatch, and anytime you have to do a gang pushback).