This guide should help you out at how to go about doing the Avenging Angel missions.
Portland - Bulletproof Avenger
Staunton - No losing weapons when busted
Shoreside Vale - Hero Garb outfit
Note: The Bulletproof Avenger is only obtainable once and should be put in a garage as before saving
Getting Started: First, you’ll need to go to one of your safehouses and put on your Avenging Angel Fatigues. They are unlockable after the mission “Frighteners”.
Where To Go:
In Portland, the best place to get an Avenging Angel is at Chinatown, more found at the Kurowski Train Station.
In Staunton, they can be found in west Aspatria near Hogs ‘n’ Cogs (Bike Salesman) and in Belleville Park.
In Shoreside Vale, they’re found in Wichita Gardens in South Side Hoods turf.
What Vehicle To Use: I'd say always use a fast and/or durable four-door car. I say that because in case you lose a few Avenging Angels during the mission, you should have spare. Here is what to use if you’re going for speed or durability.
Speed- A car like a Leone Sentinel, Sentinel, Police, Forelli Exsess, or Bickle ’76 (Finish the Taxi Side-Mission) would be perfect if you’re on the speed and balanced durability side.
Durability- A car like a Patriot, Landstalker, Ambulance, or Cartel Cruiser is great for this mission. The Cartel Cruiser is the only one of the four, I notice, that doesn’t flip at extremely sharp turns so it’s basically perfect for the mission if you want durability.
Two-Door Cars- Only use two-door cars when an emergency. If your car is about to or blew up, then run to the next closest decent car, and then drive that until you can find one of the cars I recommended. You can also probably get to a fast sports car if time is low. Remember that you’ll have to have at least one Avenging Angel along so that you can continue the mission.
Motorbikes- This may be easier to use than a two-door car because you can do a drive-by from in-front. If you're going for speed, use a PCJ-600 or Angel. Use a Freeway or Sanchez for balanced speed and durability. A Faggio, obviously, isn't recommended but if you have lots of time on the clock and you don't want to fall off a lot, you can use the Faggio since it's the best bike at turning. When you need to gain speed from trying to chase speeding cars that criminals are in, then just lean forward when needed. Only use two-door cars or bikes if you're confident your Angel won't get killed.
Time: The time at the beginning is different from Vigilante. In Vigilante, you always start at five minutes. This is a little different. Here it’s like the Vigilante from previous GTAs. When you start the mission, there is typically 1:30 to 2:30 on the clock. The islands of Liberty City aren’t that big so it should be enough to do the level. Once you reach a double digit level, time should be more than enough and you won’t have to worry about it.
Weapons To Use: Finally, we get to the action. There are different methods to kill the trouble makers. One is that they might be in an alley and you have to get out of the car and kill them. When that happens, you whip out a Stubby or Spaz Shotgun. Another way the enemies are formed is that they might be all around a blown up car on fire. If that happens, don’t bother leaving your car, just do a drive-by on all of them. Make sure a criminals don’t get to the car and drags an Avenging Angel out. No matter what you do, once the door is opened, an Avenging Angel will come out and you’ll have to come out of the car to save him before the criminal kills him. Next, the criminals may be in a car such as a Landstalker or Perennial that they stole and are driving around. Simply drive-by them until they bail out of the car and then use keep using the drive-by to blow the vehicle or just kill them. It doesn't really matter what SMG you have. Last, they might be in Freeways and Angels forming a “V” shape. No need to ram your vehicle into them to knock them down because if you do that with one of the durability vehicles I suggest, then they might just do a back flip and turn over. Plus, these guys can ride bikes so it would take a very big hit to get them down. Just drive-by them off the bike.
Criminals: The people you have to kill in this mission are criminals you see on the streets sometimes chased by cops in the street and the Bikers you see in the mission “Overdose of Trouble”. They will only wield melee weapons. They have Brass Knuckles, Katanas, and Meat Cleavers
Avenging Angel Involvement: Try to keep the Angels out of the action as best as you can if they don’t have melee weapons. You can get them to have melee weapons if they run through them but sometimes they don’t pick it up. You can try to let them get a kill once in a while but if you’re using a Shotgun, make sure they don’t get caught in the fire. Sometimes they don’t respond to a criminal hitting them so you have to kill them yourself.
Mission Difficulty: Overall, this mission is very easy to complete. The only thing you have to worry about is rival gang members shooting you while your doing business or that you don’t drive that well. This difficulty range on Portland is about 3/10 since there are no tricky roads. On Staunton, 5-6/10 since there’s this road that might trick you in eastern Staunton. On Shoreside Vale, it’s a 7-8/10 since every driving mission is difficult in Shoreside Vale since there are tons of tricky roads that you’re going to have to pause to your map for to see where they are. Plus, Cartel practically are crawling around the place.
Conclusion: The mission ends after level 15 for each island and the only ways to fail the mission is to get wasted, busted, or to run out of time. If you ran out of Avenging Angels to tag along with you, then you don’t have to go straight to the places they’re usually at because to help you, they randomly spawn at the streets you’re at.
If any errors are in this guide, just tell me. This also may be updated if I notice I missed anything.