I did this on Online but it works also on SP.
Step number 1:
Go to this Location near Franklin's house:
Step number 2:
You will see a house that has a balcony with a big piece of metal on it and a rental billboard, once you find it climb into the balcony and take cover. It's this one:
Step number 3:
While you are in cover, a group of Families gang will spawn across the street. Throw a Tear Gas grenade and take cover
Step number 4:
Take cover and DO NOT fire at them. Just while in cover, take your cellphone and call the cops. If you are lucky while shooting at you, some other Family gang members will join in. The gang members can't kill you because the balcony protects you from the shots so you are not going to die.
Step number 5:
Wait until the cops arrive and watch how the family members get killed