When you are in mid-air, press the dodge button (Z) and then the attack button (X or C) and you will do the power attack in mid-air, but you have to dodge when you're completly still. I know that it is intended (Thanks to LegitToast)but it is still very useful. Please note you have to do it very quickly if you want to do the up,down,left or right attacks. Hopes this helped and have a great day! :D