It’s bugged, you can only get past a wave if you kill the wave all at the exact same time. Been over an hour of trying for one easy quest in the mid of the base campaign on my Prot Warrior. Yay for the quests on the rails, I can’t do anything for the campaign without this one quest.
And yes, I’ve filed a ticket. It’ll be days for that…
you should have posted this in bug forum if you wanted it to get blizz attention then…
this is gonna get drowned in all the complaints about the maw and torghast 
Yep, I’ll do that eventually too. Right now I’m trying to switch to Arms to see if I can kill them all at once. Very difficult as Prot, they keep casting a shield on themselves and it’s tough to time their deaths to be simultaneous.
Yeah, just very difficult with the last wave. Tough to get all 3 low on health and their shield then get the killing blow in one strike. Tried on Arms, Fury, and Prot and no good.
Guess I wait 5 days for the ticket!
Talking about this here is ok if you just want to see if anyone has a workaround or has found something that works. The Bug Forum will sometimes have input that might be helpful.
Already a topic there, people have been posting in there for a long time and nothing. No one has found a workaround. I added my voice.
https://us.forums.blizzard.com/en/wow/t/the-blade-of-the-primus-quest-bugged/737946
Saw this on wowhead:
This quest is bugged. on Second phase when Echo’s spawn they have to be killed at the same time. In the Forge 37.76 There is a blazing breast plate that will one shot echo’s just make sure they die together.
I gave up for the night but if Blizzard takes their time I might look into that, thanks.