After you save your sister, go back to the quiz area.
Walk through the quiz board freely, then take a right up to the lair of Gruntilda.
Make your way to her cauldron so he can blast you up to the roof where you will fight here.
There are 5 patterns of attack you must do against Gruntilda to win.
1. Dodge her flying broom attacks and peck her on the back.
2. Dodge her fireballs, climb on the edge of the roof and spit some eggs at her.
3. Take to the air via the flight pad and dive bomb her midair while dodging her fireballs.
4. Shoot eggs into the jinjo statues (4 of them) to activate jinjo attacks on Gruntilda.
5. Shoot eggs into all 4 sides of the big Jinjonator statue to activate the Jinjonator against Gruntilda.
Note: make sure you use your gold feather invulnerability against that homing white fireball or you will get hit by it. Also, make sure you upgrade your health to double health and stock up on the BIG feathers and eggs before the fight using those extra music doors around the cauldron.