To strip, and Kazuma does.

"Okay... If you win, I'll... Strip naked for you?" she suggested.

"Done! Ready set go! Create Water!" Kazuma shouted, splashing Yunyun before she could react.

"Whaaaa-!? Yeek, it's cold!" she protested, being wet in the winter air. "No fair coming at me before I was ready!"

"Freeze!" Kazuma responded to her protests with another move, this one causing her to be frozen in place, coated in ice from the water on her freezing over.

"I should have warned you, Kazuma fights dirty," Megumin remarked in a bored tone as Yunyun cried out and struggled to free herself. "Must have slipped my mind..."

"Get me out of heeeeere!" Yunyun pleaded. "You win, just let me out!"

"Sure," Kazuma replied with a smirk as he approached. "Didn't even get to threaten you with a Fireball though... Oh well..." He then began thawing her out with Kindle.

...

"Alright, pay up," Kazuma smirked once they were back inside.

Yunyun blushed, but nodded. "A deal is a deal," she said hesitantly, before reaching for the hem of her top, undoing a few buttons before pulling it off, showing a pink bra underneath. She then reached behind herself and unclipped it, letting it drop and freeing her nice breasts.

She continued to blush, but seeing Kazuma's gaze on her, clearly liking what he saw, she smiled a little, getting into it as she then pulled off her skirt and panties, along with her shoes and stockings. She then straightened up, showing off her body for him. "...do you like it?" she asked, a mix of embarrassment and eagerness in her tone.

"Looking great," Kazuma grinned.