Well, if no one else is going to continue, I'll throw my hat into the ring.

Cloud would get himself owned. That's all there is to it. His one distinct advantage is that he is far physically superior to Ness. He can take a lot more damage, and can deal a ton more with his sword than Ness can with his bat. His one physical downfall is that he hasn't displayed the amount of maneuverability that Ness has in order to effectively use his physical advantages.

You see, Cloud has only appeared in a total of two games. FF7 and Kingdom Hearts. I don't know what he was capable of in KH (or if you even saw him fight anyone) but in FF7, he didn't show a ton of agility or speed. Ness has the advantage of being featured in an RPG, which gives him insane offensive capabilities (like any RPG hero) but he has also been featured in two fighting games which give him incredible maeuverability and an assortment of skills that he can constantly draw from.

If Cloud could get in close and use Omnislash, the fight would be over, but Ness would never alow that. He'd keep himself in the air and pelt Cloud with a number of psychic and PK-aided attacks, never once allowing his opponent any room for attack.

Now, like I said before, I'm basing Cloud's performance solely on FF7. I never played KH, therefore I have no idea what else he is capable of. Going by FF7 alone, there's no chance in hell that he could win. Maybe you KH fans have a strategy that beats mine. Let's see what you've got.