Get a dice and define actions for any number, e.g.

1 - kick
2 - squeeze
3 - knee
4 - slap
5/6 - she has to undo a piece of clothing

If you roll 1-4, roll again for the amount of kicks/knees/slaps/seconds of squeezing she gets. She wins if you can't take it anymore and give up. You win if she is completely naked. BTW you should be naked from the beginning.