Enable the "Bodog Stampede" code. Then, go after the gangs with it. The gun will have infinite ammunition and enough armor to take them all out. If the vehicle becomes badly damaged, enable the "Repair vehicle" code. However, you must move the Bodog Stampede once in awhile or they will pull you out. This also is very useful when taking over the storehouses because of all the armor it has.

Use the following trick to eliminate gangs during cop interference. If a cop comes by during a fight with a gang, run away and go by his vehicle. The cop will ignore you and go after the gang instead. When he is killed, resume your attack. Make sure to put your weapon away or you will have to evade the cops.