Happened to me only 1-2 times. Its pretty weird that you are getting it all the time. Try lowering difficulty/increasing magic resistance. I also had this problem but managed to survive first hit for 1300ish with my 90 vit and life point enchants,then just without trying to heal myself jumped next to the boss and used splitting arrows for instant kill.
But yeah,dunno if its same spell all around but remember i could avoid it by constantly running and rolling on the floor. There was no projectile,but boss couldnt target me that way.
