after you fought her the first time and escaped, the character said something like "can't engage her directly, got to do try another approach"
then you talk to ppl in town and figure there's a massage guy, so you check out his place and interact with his book.
after the event, main dude said something "need to drug her and the drug only grow somewhere above 3000 meters"
so you find the mountain passage. it's north of the forest a little passage among the mountains.
the path in there is annoying as hell, so I' suggest you use the save before you release the prisoner.