I beat her at level 25. Basically I used the bow, I used cover and I stayed at range. It took forever and hitting the spheres seem to do a lot of damage. As she basically dealt with Merrick, I hit her from afar. It does work and it wasn't a glitch and it simply takes time and patience but it can be done. The only glitch that I had was at the end of this quest when I deposited the item, that's when the quest glitched but when I came back it still gave me the victory.
So it wasn't a luck and it wasn't a glitch. I watched as her health went down little by little until she finally died. whether you believe it or not, it can be done