While fighting Bheithir, lure her to the right side of the platform. You will be walking backwards if you are locked on her as a target. If timed correctly, you will fall onto a little lip on the edge of the platform. From that position, keep shooting her. She will continue trying to walk toward you, occasionally breathing fire (it will not hurt you) until she eventually starts to fly. Then, a cutscene of you killing her will play.