Well, here's my big rant. Now, none of this is necessarily set-in-stone proof, but it's pretty hard to argue against one way or another.

First and foremost, let's examine a few key scenes in FF7. One, Sephiroth's original "death" at the hands of Cloud. After impaling Cloud upon the Masamune, the seemingly unbeatable Sephiroth's attack is suddenly thwarted as his assumedly inferior opponent grasps the blade of the sword and uses it as a lever with which to hurl the superior warrior into an abyss of whirling Mako energy. There are two things that this scene suggests to the player. 1) Cloud is mush stronger than you would initially assume. 2) Sephiroth is nowhere near as powerful as he is reputed to be.

Another key sequence involves the only spoken lines of Sephiroth's "mother", Jenova. After killing Aeris, Sephiroth flees, leaving behind a fragment of Jenova to finish off his opponents. Upon defeating the creature, Cloud and his party hear the creature utter the words "Cloud, you are nothing more than a puppet." This line is immediate proof that Jenova is not some mindless, destructive creature, and it is the beginning of the future revelation that it is not Sephiroth who is in control, but Jenova who is authoring his villainous deeds.

Throughout the entire rest of the game, we watch as Sephiroth behaves in an odd manner, almost as though he is not truly the one in control of his actions. The scene in which he kills Tseng is a great example of this. During that scene, we watch as his body blurs and distorts, phasing in and out of reality, exactly how Cloud's body looked each and every time Sephiroth chose to exert his genetic control over him. This is clear and irrefutible evidence that Sephiroth is not the one in control, but it is Jenova who is in command.

Every aspect of Sephiroth's transformation from a hero into a blood-curdling villain is clear evidence of this. Every seemingly illogical or unclear motivation of his is clear and undeniable proof that he is not the true villain, he's just simply the avatar for the true villain, Jenova.

Another key sequence involves the very end of the game, in which Cloud and his allies battle against a final form of Jenova, as well as two of Sephiroth's uber-destructive forms, Bizarro Sephiroth and Safer Sephiroth. There is one very specific but minute detail that many people may not quite catch during the Bizarro Sephiroth sequence. If you look closely, you can see that the massive creature indeed looks like a giant, malformed, freakish Sephiroth. However, growing out of the top of the beast's head is the torso of Jenova, using her tentacles like the strings on a marionette to control Sephiroth's actions. In the Safer Sephiroth battle, Sephiroth has taken the form of a monstrous angel, with one huge wing protruding from where his arm should be. It should be noted that the colors of the wing are blue, white, reddish brown and black, the exact colors of the four incarnations of Jenova that Cloud and his party fought against, once again signifying Jenova's control over her son.

Finally, after being thoroughly defeated in all of his forms, Sephiroth meets Cloud, face to face, in his natural state, no weird bizzarro forms, no tampering from Jenova. A true fight, one on one. And he falls before Cloud's feet after a single attack. (Make note, even if Cloud chooses to not use Omnislash on Sephiroth, eventually Seph will attack Cloud, doing an insignificant amount of damage to the hero, which will cause Cloud to automatically counterattack, which will kill Seph instantly.) So, how is it that the villain that was capable of summoning Meteor, Super Nova and killing untold numbers of people single-handedly now falls helplessly at the feet of a man that required an entire party of allies to defeat him earlier? Because Jenova is no longer helping Sephiroth. After defeating Sephiroth in his Safer form, Jenova's influence over him disappears. She is defeated and her augmenting powers are rendered null. Thus, Sephiroth proves a second time that, on his own, he is nowhere near as powerful as one would be lead to believe.

To furhter emphasize this, once Sephiroth dies in this scene, he disseminates into Lifestream energy, destined to join with the rest of the Lifestream inside of the planet. However, Lifestream energy is green. When Sephiroth evaporates, the energy he turns into is red, signifying the contamination of his life energy by Jenova's prolonged control over his mind, body and soul. Why does that signify a contamination? Because the green Lifestream energy of the planet identifies natural living creatures in the cycle of the planet's existence. The green energy signifies things that are naturally a part of the planet's cycle. Jenova is an outsider, an irritant in the planet's cycle. She is a pollutant, and the fact that her life energy is tainted red instead of green is evidence of that. The fact is, if it hadn't been for her exerting control over Sephiroth throughout the entire game, his Life Energy would have been green, because he was born naturally, not in a test tube or created from Jenova's cells. He was the biological son of Hojo and Lucretia, and therefore he was a natural part of the planet's life cycle. Jenova's prolonged influence over him corrupted his Life Energy, turning it red in the process.

So, it can then be deduced that Sephiroth's great powers did not, in fact, come from himself, but instead from Jenova's prolonged control over him. Once she has been destroyed, we see clearly and plainly that he is nowhere near as powerful as he was before. He's just a guy with a badass sword and some cool Materia. Badass swords and cool Materia are certainly nifty, but I doubt very much that they would be sufficient to kill the demonic Lord of Vampires.