I got it without really trying. I did the scene where you meet her in the village. Then I just kept doing missions for Fujimori, but also I would stop and talk to her each time. I'd tell her she was privileged, that I would be her pawn. Basically whatever she wanted, I would pick complimenting answers. Eventually I got an event where a servant was at my home and asked me to meet her at night. Then she asked me to kill Shuzen for her.
Do what the strategy guide says to do for Ending 15, but note you have to do a 100 man battle at some point (piss off any of the factions then go to that vendor) then talk to her about who would be a better ruler then Shuzen and choose the third choice.