The battle between the Grand Toad and Skoll became heated because the latter literally changed the soil that she is standing on. The ground beneath the unique boss can only be called burnt ground as that is what is being displayed when anyone uses Inspect.
The burnt ground is favorable to Skoll as it cannot be controlled by the Grand Toad. She managed to escape the Grand Toad's attacks during their previous clash. She immediately too distance and burnt the ground around her in fear of the Grand Toad's masterful utilization of the element it is affiliated with.
Skoll became so hot that a dome around her would immediately turn the rain into steam. Her fiery gaze is no longer as fully effective as before as Lycan managed to blind her left eye. She can only use that fiery gaze if she focuses on an individual.