Instructions
Play through the Night Castle level until you reach the boss fight with Windy Pants. Avoid his attacks; he will attempt to fan his stink attack toward you.
Blow into the microphone on the DS when prompted during the battle. This will blow Windy Pants' stink attack back at him, leaving him vulnerable for a few seconds.
Press the "A" button repeatedly to attack him while he's dazed, and then run away before he recovers to avoid his counter-attack. He will occasionally attack with a more wide-ranging stink attack; press "B" to jump over it, as it cannot be blown back to Windy Pants.
Blow into the DS microphone whenever you're prompted to blow Windy Pant's stink attacks back at him, and then attack him while he's vulnerable. Keep this up while dodging his attacks until he is defeated to complete the Night Castle level.