During ony today i accidently sat too long when she was deep breathing and then hit my Oshit button deterrence and it deflected all the fire as it passed over me, I tested it again and deliberatly stood in front of her as she did it and it deflected all damage again.
Then in phase 3, right before she cast her fear i hit my deterrence again and it deflected the fear and i didnt run around like a chicken with my head cut off, hope this helps you guys. It sure helped me.
If you do the deterrence for the fear make sure to cancel it right away so u can continue to DPS.