lvl 18 warlock. failed several times over 40 minutes until I found this thread.
Finally worked when I feared the first attacker without it touching the banner, killed an additional add the feared spawn pulled from the surrounding area, then let the first mob beat on my void walker until the king spawned. King actually spawned quite quickly. perhaps only a minute.
other failed attempts the banner despawned. even when i was letting the first mob beat on my voidwalker.
Still bugged as of 12/2/20
lvl 39 druid here i spent quite a while here trying all the proposed solutions here.
What finally worked is letting the first furbolg attack the flag for awhile and then attacking him and then tanking his attacks while casting a non-damaging spell while healing in between but then after about 30 seconds to a minute of that i killed him and had the next one spawn in and repeated what i just did and thats what finally made the chief spawn in for me.
Every other attempt to keep the flag up didnt work until i actually killed the enemies after a bit so hopefully this additional info could help someone :>
Still bugged Dec 7, 2020
Still bugged
lvl 20 Monk tried for days but took advice above. self healed and did non-damaging spells until boss showed up (leg sweep -stun) and (quaking palm) which turned off auto attack.
So I found a way around this as well. Being that I am a hunter, when i placed the flag, i just let my pet and the enraged foulweald fight each other, while the flag was still active. Then around 1-1.5 minutes, the chief came in. This method worked for me.
However, the quest is still bugged.
Ok the way it worked for me was thisâŚ
(Iâm level 42, Gnome Mage, completing for achievement list. So easy one shot kills for me)
Still bugged 
Edit: Finally got the boss to show up after 12 attempts of trying various âstratsâ for a simple quest ⌠with a 2 min CD I just start dropping flags on cooldown till he showed up. Thereâs at least 4 invisible flags standing on that hill .
Spaghetti code Indeed.
Itâs still rooted.
1/14/2021 it is still bugged. Iâve been trying for 2 days now. Blizzard needs to get on it
1/17/21 Still bugged. This chief doesnât give a damn about his tribe, Iâve killed so many of them and he refuses to show up.
Still bugged, canât get the flag to stay up at all. Also not really easy to stand around as an at-level mage. Trying to let the critters almost kill it several times in a row didnât help either.
Quest had bugged for me too but after an hour I managed to get the chief to spawn. I planted the flag, let the first mob attack the flag, hit the mob with my foam sword toy to get agro but not kill, and then waited for a couple mins and the chief spawned. This took many attempts.
if you stand off the mound and range them, let them aggro to and wack the flag once and start the bar THEN kill them
then it should work, I need to work on my sentences guys, sorry
Still bugged but finally did it. I was on my lvl 31 Feral Druid and what I did was plant the flag in cat form and instantly Prowled and waited till the mob started the channel then Skull Bash to interrupt and kill then instantly Prowled again. Rinse and repeat for 3 or 4 mobs till the boss showed up
29/03 Still bugged, i tried the suggestions above it what worked for me is placing the abnner and run way from it, wait until the fulborgs start casting âdestroy flagâ and before it ends, start attacking and kill then as fast as possible, repeat until chief spaws
Still bugged, no matter what I do I canât get the Chief to appear. Iâve tried every suggestion available but nothing works.
As of 12 days later. Still bugged.
Still bugged⌠
Tried the luring work-around to no avail.
EDIT: The trick that worked for me was to place flag and intercept the mob before they even start casting destroy flag, and tank damage until the chief comes.
Itâs literally been 7 months since this was posted. Blizzard has done nothing.