I spent a few minutes searching around on the web, and haven't seen anything. It seems to be a pretty mutual frustration...

ok, i think i finally figured it out, and have a very sore shoulder to prove it.

the katana is a lot like wii tennis. it's all about the right movement, with the right timing. after i got past the blue (or orange, i forget which) belt, the wiimote icon/helper disappears during the katana tests, and you have to figure out which way to go on you own. if the guy moves his sword left, you jerk left (and it can be a very subtle flick). if he goes up, you go up, if he goes right, you go right. the slash can pretty much be almost any opposite movement after that. and in most of the tests, just blocking counts, so don't worry about the slashing so much as the blocking.

I had to have a couple of beers and really sit down with the game to get it right. it's almost like drinking at a pool hall: your first game sucks, but after a while you hit a sweet spot and can do no wrong; have another beer a half hour later, and your timing is all off...