12-10: Grunty will basically run around the tower, as though she's following you. When she stops, whack her. Then, quickly whack her spirit.
Then, Grunty will repeat above for one more time.
10-8: Now, Grunty wil attempt to pound you by bouncing up & down repeatedly. When she eventually stops, whack her, then whack her spirit.
As before, Grunty will repeat this once more.
8-6: Grunty will fire those annoying spells at you. For now, just run around in a line. Then, she will try to flatten you by coming down on you. Whack her, then whack her spirit. Repeat once more.
6-4: This time, Grunty will cast a homing-spell. As long as you don't turn around, you'll survive. Then, when the spell disappears, whack Grunty and whack her spirit. Repeat once more to destroy the Mecha-Grunty shell.
3-Complete: Just because she's in spirit form, it doesn't mean that she can't play a final trick on you. She'll clone her spirit, leaving two false versions, plus the genuine Grunty. Roll into the spirit with the Shadow. Then, you'll just need to roll into her again one more time to complete the game.
Congratulations, you've completed the GBA-offering of Banjo-Kazooie!!!