Apparently, Cloud is able to fly in kingdom hearts, which could be a huge advantage.
I could see Cloud flying around and then shooting things like meteorain or finishing touch or blade beam, but those would take time to do while X can continuously shoot. Added also X has an assortment of different style weapons.. including one that can automatically aim.
Of course being the Cloud's Ultima Weapon is extremely powerful it may be possible for Cloud to use the sword as a shield blocking any attempted projectiles sent by X.
Cloud could.. for example.. while in flight, charge at X (perhaps from behind) with sword in front blocking projectiles and then get in close for an omnislash or cross slash or whatever.
X may whip out the saber up close too.. but i don't think the saber would be as powerful as the Ultima Weapon.
X may be able to dash away in time as well though.