The fire wouldn't affect underground, and we're in the land of waterfalls, the land is rather moist here. She'd only need to overturn the top-soil to prevent herself from being affected by the scorching ground, a simple feat for someone with the Earth element.The scorched ground? Yeah. Good luck burning XD
Which the coordination would leave her open to be hit by the fire attack Originally.
Eh, unless of course, Baiyan's genjutsu has something to say about Reaver's aim.