Thank you so much for this, i'd given up ever getting cool master after trying all the previous posted methods. Most frustrating since it was the last achieve i need to 1k the game. Your method worked flawlessly although it did take me 3 goes to get the timing down.
As mentioned above, its important to press B and immediately B again, practically a double tap, then wait whilst Saki pulls back her sword and just before she starts to swing forward press X. Took me a little while to work this out in practice, I was doing B too slowly so just doing a normal X attack. Once you've got it worked out though you'll know as she does a horizontal slash, followed by a vertical jump down slash with that white-ish trail following her sword.
Anyway thanks again.
Although i'd forgotten that you needed to do this three times to get cool master. Looking at the quest page it appears the old lvl 11 method does work as i've got two of those and one lvl 15 'dot'. Oh well guess i wasn't as bad as I thought