Dont know how mines got fixed. Gave up after 30mins and did the warfront quests instead. Came back just to see and all of a sudden she was killable.
I just did this quest and I was able to defeat her, then Nathanos comes, talks, she says, “no…” and disappears, and the quest still doesn’t complete. So, progress, but still bugged.
Bugged on every realm it seems. There’s a lot of people trying to do this quest, so it’s very difficult to make everyone stop fighting and reset.