I can't Bamboo Rain?

Few tips:

1) As many have spoken, hold the "up" first, then press ctrl and alt together while holding "up".

2) If you have skill/pot or w/e on ctrl and/or alt, use some skill first, and during the animation, press the combo.

3) Stay afar from the boss, it hits the whole map(not just the whole screen), so if you fail you won't get killed as quick.