How do you fight Greygnarl after the game ends in Dragon Quest 9?

If you have beaten the other bosses such as Tyrantula, Garth Goyle, Great Lizzier, etc, then you should have all the Fyggs that you need at the moment. Stella will instruct you to go to the Starflight Express which is at the Blue Tree near Alltrades Abbey. Go back to the Starflight Express.