He holds himself back from the full thrust of his fire's power. He could amplify the flame, intensify the heat burning at its core, but he doesn't want to make the challenge impossible for her. Like he said, she's already at a disadvantage, what with it being the middle of the summer. Still, his fire slowly but steadily eats through a chunk of her ice.

"You're not weaker," Wilhelm shakes his head. "You just haven't practiced as much. It takes time."

He's not used to thinking of himself as powerful — it hasn't crossed his mind at all, until this moment when Sansa calls herself weaker. Powerful feels like a far off point on the horizon that he's always striving for. After all, it was the raw desire to never feel so weak again, after what he endured at the hands of Josselyn's acolytes, that pushed him to sincerely dedicate himself to practicing magic last year.