First off, Sepiroth was part of the very first Super-SOLDIER task force that was formed to bring Shinra Inc. to the Promise Land. He weilds an extremely powerful Masamune blade that takes down Dragons in a mere swing, and his swordsmanship is next to unmatched. His powers were considered unnatural by those who knew him, and the other Super-SOLDIERs.

It was in an attempt from Sephiroth to take the Jenova head out of a Mako factory that Cloud impaled him with his Masamune, critically wounding him.

After that incident, Sephiroth's body was sealed in a Mako chrysalis, which made his psyche extremely powerful, most likely from the Jenova cell mutation. During this state, he found that he was powerful enough to come together in a physical form, of which any shape he could fathom was possible. Meaning basically, he can fly, appear transparent, make himself almost ten times his normal size, and travel through the ground. It was through this process he was successfully able to retrieve the Black Materia and summon Meteor.

In this fight his psyche will come greatly into play to confuse Dracula.

Also, when he casts Meteor, the time guage in the game halts, and won't allow another attack to be maneuvered during that time, which means that we can either say (1) He can cast it, fight it out with Dracula, and when the time comes for the spell to be ready, it hits, or (2) No fighting occurs until Meteor is processed and hits it's target.