If you are having trouble defeating the Avatar of Balthazar, clear out the guards first, then run away. Then attack as soon as she changes back to her original form. It's much easier to kill her while her form is recharging.
The Acolyte sometimes spawn with a mob that is without a Kournan Priest, which makes the task of clearing the mobs much easier.
Another way to defeat the Avatar of Balthazar is to have a tank supported by protection monks. 55 Monks or 130 Dervishes work excellently in this regard.
Another way to defeat her and any other dervish boss, is to bring the asura title track skill Pain Inverter and a MM with melee minions. After several swipes she will probably be dead. At R10 this works very well.