I've been trying to get this guy to spawn for his Taelosian Geomancy Stone Eril drop.
He popped today, same location as the previous poster. No stone though :(
Second spawn on the right in the first hall as the others mentioned.
I clear between 15-20 mobs to get all of the named (or their PHs, its random) to spawn in the trials. It doesn't seem to be a fixed amount but always in between there. The named or his ph can be seen from the first room in the hall so it's easy to tell if they have popped yet.
Named pops are very rare. Rigara spawned on my 13th instance and I haven't seen the Sludge Eating Fly even once yet. I grab a new instance after spawning either him or his ph, since the next respawns will usually have a lot of unsnareables when added to the first batch I skipped.
Muck covered fly and Stonemite are the only two mobs which are unsnareable in the first room so I usually don't kill them.
I can do the fly but the stonemites summon so I avoid those at all costs.
Soloed him as a 65 necro, he was pretty easy once the first room was cleared. I pulled him to there. Took 10 tries to land slow on him, he has high MR.
Undead so slowable by a necro. His max hit is about 1100 but he usually only doubles. Sometimes he triples but those are rare.
Summons. Partially slowable. Has about the same HPs as the other mobs. Enrages.