When doing a gang war when you arrive and see all the gang members about to get of there bikes and shoot you run to a safe place where you are protected from there bullets and duck down behind a wall. Now while they are fighting with your boys and you are hiding do this.
Firstly input the lose cops cheat which is done by dialling 267-555-0100 this will appear under cheats and can be reactivated via your phone by going into that menu and selecting it.
Now you need to activate the one star wanted level cheat 267-555-0150 this will give you a one star wanted level. Access the cheats menu again and select the raise wanted level cheat and hit this 6 times. Wait for loads of cops to get near you and quickly activate the lose wanted level cheat.
What will happen is the cops wont go after you and when the dumb gang members pulls the trigger the area full of cops and swat team will go after them and a fight will break out. Let them fight it out by that time the gang and cops will be wiped out or there will be very few gang members left.
Take the remaining gang members out and easy win. This works well when your in an area where the peds are armed. Try this and watch how fun it is to see a massive fight break out. I have had countless hours of hysterics with this trick on the Lost And The Damned.
Its even funnier seeing a cop car scream up to the gang members and two cops coming out firing back.
Also its best to wait for the entire area to be full of cops before you lose your wanted level that way the war will be much better. If you dont move the pad for a while the cmaera will zoom right into the fight and youll get a great view of the fight. Just repeat the process if the cops or feds dont win the gun fight.