sephiroth was powerful but i think that was him just using his natrual ability and manuplulating the jenova cells within him. he was going to become a god by obsorbing the entire lifestream thats why he sumoned metor so the planet would gather all the mako in one place, then he was going to use the empty shell of the planet to sail the cosmos like jenova before him repeating the process with all life bearing planets he came across