Stuck on quest due to bug

Hi

I’ve been leveling Legion tailoring lately and now im stuck on the quest where I need to clear Return to Karazhan dungeon.

When I come to the opera hall boss all the mobs gets stuck on 1% and I cant kill them, it goes on forever it seems.

I tried the toy whole body shrinka and it took forever to get its hp down. I even tried going naked against the boss and hitting it but it still bugs out at 1% health.

What am I gonna do? This bug have been going on for years reading the comments. And I cant complete this quest without clearing this dungeon…