Argent Vanguard/Valley of Echoes-area is horribly bugged

I hit level 78 on one of my alts and I went to Argent Vanguard to ding 80, but the last quest, where you need to kill 100 of those spiders and 3 of the dragons (The Last Line of Defense) is stupidly broken. The mobs you're supposed to kill are already dead so you cannot attack them, but they can still attack you and once you aggro them, there is almost no way of deaggroing them unless you want to run for a few minutes.
The other NPCs in Argent Vanguard has despawned if The Last Line of Defense is the only quest you have left, so you cannot buy any reputation items or anything.
Abandoning the quest doesn't change anything.

TL;DR: The Last Line of Defense quest is bugged, quest uncompleteable, which means NPCs no at all except goblin questgiver.

Edit: Spelling error

The quest is working fine.
There are still some small issues with phasing though. If you are in the wrong phase, i.e. the mobs are all dead, fly out of the zone and back in. Repeat until the mobs are alive (1-3 times should do fine) and then mount the cannons and shoot them.

The thing is, the mobs that are needed to complete the quest are already dead, not just chilling laying dead, they're aggroable and are attacking stuff except that they can't be attacked themselves.

Its a phasing issue I just do what densi77 says.

I cant even start the quest chain so i have no idea wtf is up with argent vanguard

The bug wasn't there before, but when I leveled my priest there were dead mobs killing everything and the quest where you need the cannons to kill mobs wouldn't work.

-The mobs you need were already dead
-The dead mobs had killed the cannons

It's not working fine, but it's doable.
Just relog a lot of times and kill them (most of them have 1/65000 HP so you can one-shot like 6 in a good group) and relog more times for dragons.
It works but it's just a pain to do it.

if you go to a botom of the vanguard, there is spawning point for those spiders. they have 1 hp and auto-aggro you. I activated retribution aura and stand there. 77-80 in 20 min :D