You can play this achievement a couple of ways. You can be utterly lazy and look up the guides that offer specifics, or you could look up the less exploitative guides that simply give you the name of each target.
I opted for the named target strategy. It was still a challenge and took a while, but their clues are too obtuse to play as designed for me.
It can also get a little awkward if there happen to be other players in the same area as you running the quest. One of them kills the NPC, and you have to hunt around a bit to see where it re-spawns.
On the other hand, make note of the NPC's you frequently encounter lying dead in the streets. They're often the targets -- take note of their appearance and what they're wearing; you may see them up and walking around moments later. If they have one white eye, they're a target.