Hold down back sweep then coup droit, the timing is pretty tight you have to do it as soon as you are able to, memorise how long it takes to charge and observe the animations.
Its one of the hardest challenge ive done, up to 18-21~ with 10 characters, most are not that bad though. Ive tried to practice it in training mode and cant pull it off consistently. Im on a pad so i dont think ill play mitsuru for now.
About cancel you need to hit the opponent first then you can cancel the rest of the animation with a move, not all moves are cancellable.
Here a list of what you can cancel
http://www.dustloop.com/wiki/index.php?title=Movement_%28P4Arena%29#Cancels