This worked for me.
A slight difference:
I pulled the first monster to the soldiers, then the soldiers and the monster into the cave . I did NOT kill all of them - juts left the game. After re-entering the wall was not attackable initially and the quest worked.
After like 10 relogs and clearing the event outside like a dozen times with no avail
What worked for me is abandoning the quest and then picking it up again from the camp
I did the event outside before walking into the cave
When i talked to the NPC everything proceeded normally
Still broken. Typical blizzard just ignoring anything that isnt microtransaction.
Still buged but this worked like a charm
same, same, same, same…
Can confirm this is still a valid workaround! Appreciate the help!
next day worked for me. first try and doing nothing special
Nothing worked
Still BORKED
Abandoning and reaccepting worked for me
Doesn’t work…abandoned, restarted, changed realms…they don’t follow. Bugged,
Even tried to exit the cave, clear the event (if there is an event),quit the game, start the game again, then enter the cave again,but didn`t work.
Yes. I experience the same problem. I was told that the quest is bugged when there is also helltide. So I canceled the quest, logged out and started the quest again without the event. Same problem. They just stand still. Below video shows how it’ supposed to play out:
Any ETA on a fix, I have tried multiple times to complete this quest, and I am still unable to.
I’ve gotten none of the above to work. I would hope this gets fixed after such a long time.
After trying everything, this one worked! Thanks Caleb!
same situation, trying the caleb method at the moment. As pointed here before, there should be a button to report bugs.
Still an issue… Tried the above methods for a while but somehow wont budge.
I also have this issue. Tried all the methods listed here nothing has worked for me.
Looks like i have to wait for a patch and see if it will work then.
I also have this issue. Tried all suggestions:
Guess waiting for patch is the way to go @Blizz! 
Bugged for me too, I have tried the following:
Reset the Dungeon - numerous times;
Abandoned the quest & restarted it, many times;
Changed World Tier;
Abandoned Quest, changed World Tier & retaken the Quest.
All to no avail.
I’m hoping it gets fixed in the next patch.