You don't need LoS to kill the void adds. This just shows that people aren't used to the affix yet.
Firstly it's been nerfed so the stacking dmg is now on players when they get hit, not on the mob.
But even before that, you can survive the first 2 casts without being oneshotted. The first cast took about 50% of my HP without fortitude. The second cast would take 75%, the third cast would probably be fatal without a good defensive damage reduction cooldown. You can simply zerg the void mobs down before their damage becomes unmanageable.
Also their cast is a 60 yard range so if they are surrounded by other trash, you can pull the trash 60+ yds away from the emissary. The emissary travels realllly slowly due to her chain-casts so you can easily kill the pack safely out of her range, then finish her off last.
Prior to today's hotfix this was still awkward as she'd gain her max 5 stacks dmg buff on her cast so only ranged would be able to run in between casts to shoot her. Now that the debuff goes on the players instead you'll be able to run in with 0 stacks and zerg her as I mentioned above.
So much in WoW is actually a lot easier than people think if they just stop, question the mechanics and develop a smart strategy rather than just assuming they're fucked.