Assassin's Creed Syndicate - Shoot the messenger perk

The best missions to do it are the Gang Stronghold missions. You don't need to shoot the messenger, just kill him after you notice the eye above his head change to yellow.
Sometimes he will attack instead of calling for help - when this happens it doesn't count.

[@WeskerKTH]

Many thanks for this! I found a method to speed this up even more! If you are replaying this stronghold (Blue Anchor Alley from the Progression Log menu), you spawn very close to the two lookouts. Sprint right in there and kill the first one (after he spots you, obviously), sprint right back to the other one and kill him. There is no need for stealth or for defeating any of the other enemies so it literally takes only seconds. Once you have taken the two lookouts down, let the remaining enemies kill you. This will put you right back at the start of the stronghold, which is much faster than quitting the stronghold replay and then launching it again (just one loading sequence instead of two). Hope this helps! Thanks again, and happy gaming!

[@BenHerbivore]

Thanks. Did all of these in like 10 minutes lol.

[@iratecnologia]

Very nice! Thank you (brazil)