Just done it and got the gem.
When the event starts you need to pull the boss out and kill it before the rock blocking the entrance is cleared. Stand near the rock as you kill it so as not to make it charge out and reset.
Can confirmed it really worked, I got all of my gems by doing this.
Still broken, weekly reset didnât help. Today for me: event randomly appears and disappears. Not sure if other playersâ actions (like pulling the boss in some specific way) are causing this bonus objective to randomly disappear
Or (and bare with me as itâs a bit tricky) Blizzard just fix it! Ikr 
itâs fancy kek lol lmao
For anyone reading this I found out a reliable way of overcoming this s***y bug. You only need a decent tank and a healer. Steps:
- start the event
- drag the boss out
- tank him between the two sides of the entrance to the cave and make sure EVERYONE is standing at a tank so he can only charge into a wall and not out to reset
- only the tank is doing dmg and even then only when he needs to heal him if you have more healeers you can simply stand there and get smacked
- someone runs to start the event again
- wait for the event
- KILL IT WITH FIRE!!!
This works.
And it works to kill him inside tanked in a corner, got my two citrines last night with these two methods, after countless attempts wich ended up in failure tho.
Being in a raid group might or might not help, several times these zombies both part of group and in open world kept killing him and ignoring everything that was said and thus cause it to fail.
I really canât understand why they havenât fixed it, what is causing their lack of response. Strange that when they take money for subscription they donât have this problem, everything works there.
Needs to be fixed asap people are just getting nasty and abusive for literally trying to do the quest.