All roaming trash mobs are potential placeholders. Approx 57 roaming trash in Dagnor's Cauldron, not counting the 4 static orcs and 4 static aqua goblins. Dwigus is also on the roamer spawn table. I got tired of hunting the roamers both underwater and above water, so because they roam the whole cauldron, I just stood on the center island and let them come to me, picking off everything that walked by. Using /pet attack is handy for this. It took a few days but I spawned all 5 Hunter mobs that way. Tracking is also useful to see if any named mobs spawned.
I spent about 60 hours accomplishing this achievement with my 80th level ranger. If you could get a ranger or three to help you out tracking the whole zone you could probably do it faster. During my long hours of swimming around killing all the roamers I came to the conclusion that they are all place holders for each other and killing any and all of them will spawn the rare mobs. With the exception of Barnaclebones, who seemed to always spawn near Kedge Keep, I noticed that all the rares can spawn at multiple points (probably any spawn point). Jetsam and Squallslither spawned the most frequently, followed by Barnaclebones. Flotsam took a bit longer to spawn. And finally there is Bilge Farfathom, who is a nightmare to spawn (particularly solo). Good luck, I hope this helps.
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Maulis Thefool, Ranger of the 86th season on Firiona Vie...
I swam around underwater killing everything I saw for several hours each day for 4 days. I spawned Barnacle Bones 12 times. Flotsam 3 times. Squallslither 3 times, but did not get Bilge or Jetsam at all. I spent the next 3 days just sitting near the Fab Four and checking track when I had time. Barnacle Bones popped 4 times in those 3 days. Today ( my 7th and last day) both Bilge and Flotsam popped. Thanks to the normal mechanisms in the zone of mobs being killed.
Doing this hunter for the second time because I'm a a glutton for punishment.
It seems the majority of the mobs all share multiple NPC races on their spawn tables as I was able to hunt to extinction rats/snakes until the majority of the zone was just goblins and nothing else was respawning. So RIP easy way to get Flotsam now that he is my last one.
Bilge Farfathom is an ultra rare spawn and it can take weeks to get him so he is the bottleneck for this hunter achievement. He does spawn underwater, north-west of Conium Darkblade and has a fast SoW buff on him as he is a shaman. Conium is looking into the direction of his spawn point. The spawn point is not that important tho as he will run away quickly from it after buffing himself.
The other mobs are rare spawns. Just kill everything in the zone and concentrate on the central isle and the underwater areas. At the zone borders you won´t find many mobs to kill. In this zone a tracker will be especial usefull. Filter by alpha or standard mode where new spawns appear at the top of the list (standard).
Its seems like rats are placeholders for Flotsam and Jetsam; snakes are phs for Squallslither; skeletons are phs for Barnacle Bones; and aqua goblins are phs for Bilge.
GL, this is the most annoying zone for the Hunter of Faydwer achievement.