After Sequence 3, you get the blunderbuss. Many people choose to have lots of soldiers chase them to a choke point and then blast the group. If that becomes difficult for you, there's a possibly easier way that doesn't require the cat-and-mouse game.
There's a restricted area (see map and video below) on the Port-au-Prince map along the southern part of the shoreline, directly west of the south-western viewpoint (not the small square at the viewpoint). You'll see a platform with several cannons and near it are lots of soldiers lined up nicely in a row for you with another soldier barking orders at them. While staying out of the restricted area (I approached from the beach) but close enough to line up a shot (make the soldier line as narrow as possible but keep each one visible), blast at the 5 lined-up soldiers. You should kill them all, but then you'll be attacked by a few soldiers near the area. Easy enough for your superb skills, though, right? ;) The achievement popped for me several seconds after the blunderbuss blasting.
* I already had the achievement before I recorded this clip. You can see 6 corpses on the ground when I walk up to them: 1 on the platform plus the 4 that were bunched up, then the last guy I shot later
If you do not see 5 soldiers lined up like I did, then either reload the area, or you might be able to catch a slave convoy passing by. Add them to the soldiers near the cannons and that will give you enough soldiers to attempt the other way and get them mad at you for a chance to group them up for a blunderbuss shot. (thanks z 4assed monkey