I managed to kill her. The strategy is to use mirror stance to draw aggro so your companions survive for the first turn then have Brint/Brienne use by the horns and Cait use shadow strike (smite evil does more damage but is significantly more likely to miss). Whenever sure shot is available on the champion use it and when it isn't use the champion's other powers in this order: 1. assassinate 2. celestial smite 3. smite evil. This was done on dark mode and had a 1/4 success rate when I got the proper first turn.