Can't damage Balballa

During the Trail of Corruption quest when you enter the prison of the disgraced. During the boss encounter my Witch with 3 Skeleton Gas Snipers and flame wall with SRS becomes unable to damage her. The gas she lets out continues to grow and she becomes immune and stationary.

From what i understand the fight, when she does that gas thing, u have to do 2 things
1. Go near her(not minions) to make her slam the ground
2. Dodge the slam
3. No more gas, more of the teleporting around

Thanks, I'll try that.