How do I beat Yuffie in round 10 in the underworld in Kingdom Hearts 2?

I wouldn't focus on Leon right away. First focus on Yuffie. First and most importantly, have Block eqiupped to Sora, that'll help defend againest the stars and it causes her to stop attacking when they deflect and hit her back, letting you hit some combos. Don't worry about Leon attacking you, but just in case have Cure in handy at all times, And don't be hesitant abouit using Fire once in a while too when far away.