Just for the record. No need to fight her >>at all<<. Just press STRG / CTRL to go into attack mode and whack away at her while she is still bound (I shot at her from the highground with a crossbow). Fight won't happen and you get normal experience (about 57k, I was at lvl 14). Quest progresses normally as well. Tried this with multiple saves (where I just "executed her" as described and where I fought her normally. Same results in terms of quest and EXP). Should work flawlessly.