Duster V spawns in the castle and goes past the PoD and runs back twice before despawning in the castle. He alternates spawns with Cleaner V and Scrubber X whom you do not want to touch, let them run the same path and despawn. There is a couple minutes between each of the spawns. You also need the right note to get Duster X to spawn and then you need to be real quick or have a friend in the right spot to kill him.
Duster X only seems to spawn after turning in the note to the Gemchopper merchant. You need to get to him fast before he vanishes. He's seen inside Akanon starting out near the tinkerer.
When attempting to get the Bootstrutter Framed Backpack, I happened to see a clockwork named Cleaner V spawn on the exact same route Duster V spawns on. I checked the con, and it showed green. So I figured, hey, maybe the quest info is wrong. Shouldn't be too hard to kill, right? I quickly discovered the magic of "broadcast distress calls." These things (and, I suspect, all the other clockworks in Ak'Anon) have a HUGE aggro radius. Clockwork BANKERS from the other side of the city were running me down before I lost consciousness. Moral of the story? Respect the clockworks... even the little green ones.