In original vanilla WoW, and on popular vanilla wow private servers, Theradras was soloable by warlocks and hunters due to being able to kite her between dust cloud casts.
It seems that this is no longer possible due to the fact that Blizzard seems to have changed her to no longer cast dust cloud when there are no enemies within melee range.
If you try and counteract this by sending your pet in to melee to trigger dust cloud, she will hit it with melee + thrash + fear + dust cloud and kill it immediately
This effectively means that hunters and warlocks can no longer solo princess runs like how they did in real Vanilla, which was a premier gold farming method and one of the reasons many people actually decided to play or alt a hunter/warlock
I truly hope that this is an oversight by Blizzard, and that it will be fixed in the near future, and that this is not an intended change to prevent people from taking advantage of one of the most popular gold farming methods in vanilla WoW.