Instructions
Enter the door on the top right of the screen and Yin-Yarn will attack.
Defeat the four enemies next to Yin-Yarn and he will give you access to four Buttonflies and two arrow Waddle Dees, which you can grab with "1" and use to attack Yin-Yarn.
Wait until a few other enemies fill the screen. Find the Chilly enemy and use "1" to grab it and throw it at Yin-Yarn.
Look for the Tankbot that drops into the game while Yin-Yarn transforms into Mega Yin-Yarn. Use the Tankbot to transform Kirby, then aim all your attacks at Yin-Yarn's mouth. Keep it up and you will eventually defeat Yin-Yarn.