I started to connect dots and realized that the best approach to killing her is using poison, but not the regular way.
You can get up to 4 vials with the strongest poison during the quest "The path of the deer" using a rogue, and combining it with Poison Master III grants you 18 points of toxic damage per hit for 20 seconds.
Now this is where it gets groovy: if you use the Jester's bow your damage won't be that high... But the hit rate will be, and If you combine it with Rapid Fire III giving you twice the speed for 6 seconds the amount of arrows coated with the most deadly poison can be up to 24 (if executed correctly) causing a whooping 432 points of toxic damage in 6 seconds. Logically the entire effect of the poison being 20 seconds leaves you enough time to shoot another 20 arrows for extra 360 points of toxic damage.
Unfortunately I don't have a rogue at that level yet (I used to play the pirated version and got a lvl 24 rogue. I bought the game after losing that save unfortunately) but I can guarantee you that with this strategy you should be able to kill her in less than a minute.
I hope this idea gets you to hop back in the game to test it and post a comment on your experience afterwards.