I finally bothered to learn the name of the school where Yumi and co are at. It’s 死塾月閃女学館（しじゅくげっせんじょがっかん）or Shijuku Gessen Jogakkan in romaji. The shi means death, juku means cram or private school. I think Gessen is the actual school name and means something like ‘moon flash’. Jogakkan means girl school (jo means woman). You might know school is gakkou, gakuin or gakuen in Japanese, so what is this gakkan? I might be wrong (it could be simply another variation of gakkou) but I’m guessing it’s a school where you also live in there.
In any case, I managed to get the best ranking for the Gessen’s final mission on Hard with every Gessen girls. On the last mission, they fight Asuka only but two times.
The general strategy to get the best ranking is really to just keep parrying until the ninja scrolls are full, then undress to yin mode for maximum damage, and spam ninja arts. Also, limit break when trapped in combo to avoid further damage. The reason for not undressing to yin mode earlier is to not absorb too much damage when trying to parry.
Yozakura’s second ninja arts in my experience deals less damage than her first one, which is kinda weird (maybe I didn’t activate it in the right condition, or the final boss is simply good enough to escape in the middle of it ^^;)