Successfully complete Leon McAffrey's Crazy 69 mission to unlock the Avenging Angels side missions. Go to the same park where the mission took place dressed in the Avenging Angels fatigues at night. Look for another Avenging Angel (they are dressed the same). Press R3 near him to start the vigilante-style killing spree. In these missions you will fight various groups of hoodlums around Staunton Island. The best strategy is to run them down in a car, or use a bladed melee weapon to cut them down. Using Molotov cocktails, grenades, and other weapons capable of taking out multiple opponents at once. If your Avenging Angel friend dies, you must find another one within the time limit or the mission ends.