then you must be VERY skilled...
No he isn't. He's being smart. The best way to deal damage to Sephiroth is to let him hit you in the air, and then when he gets close enough, use Aerial Retaliation + Berserk Charge. This takes more damage off than Trinity Limit; but it lacks the factor of invincibility (not that Sephiroth can react for awhile).

So uhh, What does Triangle, X and Circle do when Sora uses the Trinity Limit???
X = Attack (used 99% of the time with a 100% chance of statistics being 0% wrong, but also at the same time, only 101% correct)
Circle = Jump/Aerial Dodge (unused, used only outside of battle)
Triangle = Break/Reaction Command (unused, used only in severe circumstances)
Square = people use this button!? WTFAKU! ROIMAIGAWD!

This button pattern also applies to the rest of Kingdom Hearts II.