As soon as you get the blunderbuss and purchase ammunition for it, head towards the beach. There is a small fort at the lower left corner of Port au Prince, which shows on the map as red with a chest in it, directly west of one of the viewpoints. Walk in there and about 15 guards will attack you. Once they are all around you, run out one of the exits just so they are all on one side of you, then turn around and press . I killed 9 guards with one shot using this approach.
This is also an optional objective in Memory 9 where you are required to kill 5 guards at once with the blunderbuss, and it is best done in this same location as this is where the governor runs to after he spots you.