Keep in mind Aeda is a TANK despite being a Warlock. You have to treat her as such (her Alliance counterpart is a Prot Pally).
Vivianne does a monstrous amount of damage no matter how many mobs she's attacking.
Tormmok is questionably useful at times, but he is decent at single target, although Talonpriest Ishaal is way better than him at it. Both get smoked by Vivianne.