Now, as for Cloud and Co. defeating him, that's just how it has to happen. He's an antagonist, and Cloud and Co. are the "heroes". It wouldn't make for a very good ending to the story (for some people) if Seph had been victorious. He also falls victim to his own arrogance and pride, which was his downfall in AC/ACC. Cloud could barely hold his own during that fight, and it was only when Seph pushed him to his limit that he was able to take him down.
I don't care if you think he's the worst villain in FF, because I don't think he is meant to truly be "villainous". I think he is a rather good antagonist though, and yes, I believe there is a difference between the two.
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