Roaming trash in the Gorge are placeholders, there's no set spawn point. During lazy times, I just stand on the path and kill everything that walks by using a pet. I camped the southern 4-way at 150, -50 and eventually spawned all 4 rare mobs.
I've heard that King Xorbb is the rarest, but so far I've seen King Xorbb x3, Yymp x4, Spinflint x3 and still no Brahhm. Probably just variance and they all have similar chance of spawning. Seen Qlei twice... and he's not even on list.
This is really distressing for me to read, lol. I am about 18 hours into working on this achievement and I have seen
Brahhm x12 (at least) Yymp x4 Spinflint x1 King x0
Kill me -_-
Edit: Finally got him after about 26 hours. Ended up getting Brahhm 4 more times and Yymp 2 more times before King Xorbb spawned. Never doing this achievement again.
For once luck was with me! King Xorbb spawned after only 8 hours of killing in this zone. during that time Spinflint spawned 6 times, Brahhm 3 times, and Yymp the Infernal once.
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Maulis Thefool, Ranger of the 86th season on Firiona Vie...