Wailing Caverns is STILL bugged, Mutanus - last boss, needed for the achievment - is not spawning at the end of that long run to all the other bosses.
There were many reports on this and for a good while now (see US forums), still nothing has been done.
Pls fix.
And here’s to what I did on my second, sucessfull run:
was near the friendly NPC all the time, never going further than a couple of meters from him
left some regular mobs on the way to last, let them hit him
let every wave in the last chamber hit the mob, while he had his final cast
Dunno if it helped summon Mutanus or it was just a coincidence, but thats what I can offer.
Good luck! Doubt Blizz will do anything about it soon or ever.
I suspect it might be related to player level, but that is just my theory. If the boss about to spawn does not detect any nearby players (because at level 80 your aggro range is very small for him) then the system will consider that the group has wiped and will not spawn the boss.
To test it, try going in there with a level-appropriate character, if the boss does spawn every time with that low level character there, then my theory might be correct. If he still does not spawn, then it is something else.
Sis you get boosted by a lvl 80? Ive noticed that if you kill the spawning mobs too fast he bugs out. Kill then 1 by one slowly like you would have done in a normal group and he worked for me atleast.