[Arson is just your solution to everything, isn't it?]
The bat girl chokes and splutters as the fur around her head catches aflame and she turns into a small bonfire. The lighter fluid that splashed lower catches alight as well, but her Kevlar vest is flame resistant and the small amount of fuel isn't enough to set it ablaze. The result, from Stephanie's perspective, is eerily similar to Ghost Rider.
Rose, for her part, sees a sudden light spring into being. It's smaller than a torch, but clear enough to allow her to see Stephanie, lighter in hand, standing over the bat girl, down on her knees, her head nothing but a sphere of flame.