The Charged Shot X1 - X4 is not comparable with the one X uses in X5 - X6 in certain armor. It is blockable as in, you can stop it from moving further, but by doing so, the one who's defending will take damage, unless he's using a foot thick shield. (Sort of like Blast damage which stay for a few seconds)

What can Cloud do when he's flying? He might be a master in close-ranged battle, but...leaving the ground and keeping a distance from X? Say, when Cloud is in the air, X could use Meteor Shower (Charged Maverick weapon in X6) and...I don't think Cloud would be able to dodge them.

U-Buster in X6 makes ALL of X shots (Including Maverick Weapon) charged. It takes no time at all.

Omnislash is a combo (Combo of slashes), apparently. As I have said, X has this Damage Barrier which will activate whenever he gets hit. With D-Barrier (a part), the duration of the invulnerable doubles. After the first hit gets in, the several follows will not damage X. That would account for any combo performed to X, actually.

P.S. There was an arugment about the ability to use magic for FF characters awhile back, by the way.