It was stated in Jimminy's journal that LS was able to master the keyblade much better than Sora, which mean Sora would have been defeated if he went up against LS alone. And we don't even know if Sora even technically defeated LS. The fight may have just ended because LS realized who he was really fighting.
Thank you. Proof that Sora manged to beat someone more expirenced then himself.
Jeez if he beat LS without know a Master to teaching him. I'd hate to see what he become if he ever had a master to train him.