Sam can fly, so it won't be too much of a problem to keep in close range and fight the harpy with her swords, rather than fire (don't want to turn the wind into something even MORE dangerous after all). She'll keep harrying the harpy at close range, making sure the wind from the fight doesn't blow Stelle into the pit. And when the harpy's fully focused on Firefly, Stelle will come in from behind with her bat and smash the sapphire.