I have completed that on 2 characters so far. A lot of the early issues were from people not understanding the mechanics involved in the Group Challenges.
There is always a trigger of some sort, either by killing certain groups of mobs right in the area or manipulating an object nearby to get the Group Boss to spawn.
As for the other Public Dungeon bosses, the spawns can be tricky. Ones that have multiple bosses will often only spawn a couple at a time, which is random. Basically you may have to run around and kill bosses repeatedly until the one(s) you need finally spawn. Some of them do seem to take longer than others and Village of the Lost in Coldharbor is particularly challenging to get around in.
I found that doing them as early in the morning as possible seemed to have the best chance of having more bosses spawned.
This may help you out some
tamrieljournal.com/public-dungeon-group-challenge-conqueror-guide/
Also if you are on PC there is a Public Dungeon Champions addon that shows the locations in the dungeons. (warning some of them have been switched around for some odd reason)
Mighty_oakk wrote: »Just to clarify. Its for public dungeons only and the achievment is for killing all the micro bosses in every dungeon (avg about 5 per).
Not related to public group dungeon event achievment.
Couple of these just seem to be missing. I have never seen anyone with the title on ps4 either.
That being said I could just be blind