MasterX, in Kefka's "average" form, befire he got the power of the statues, he was able to kill 8 espers at once during the scene where he burned down Thamasa.

Since Sephy's "average" form was before he got released from the Northern Crater, (Remember guys, the "real" Sephiroth is the one sealed in the Northern Crater) he was able to control Cloud and kill the Midgar Zolom easily, but it showed nothing beyond that.

Controlling a guy and killing a giant snake vs. killing 8 extremely powerful magical beasts.... yeah.