Level nindy? Bah! I beat Sephy at 78 using the Lionheart(though I think many have beaten him with lesser stats). It's not hard when you know how to beat him. There are some really good guides on the eyesonff webpage as well as Game FAQs.

Anyway, I just do the Hades Cup, or battle at Hollow Bastion even. It's well worth it. It makes it all the more easier to beat Ansem (which one could beat at level 50, I'd immagine.)