You overestimate Sephiroth's power. The only thing that really showed that Sephiroth had pretty good power was the fact he could surpress Holy.
He could controll Jenova, was absorbing part of the Lifestream in the Crater, AND in AC, he became a God.

If he was a genius he would've counted on his barriar being broken through. He had no secondary plan, he counted on his barriar and when it fell he had no backup plan. Nothing to fall back on
He had Jenova Synthesis to protect him. And he have a "barrier" that protected him(when the heros reach the core of the world, Sephiroth's "barrier" made them float in the air, and they couldn't move their bodies, and when they tried to aproach him, they are threw back by a green energy).