I think Tav has this one. For me, I felt Sephiroth was only difficult because of the god awful camera which Sephiroth exploits to get off Sin Harvest/DHC or to utilize his sword combo by always teleporting in a position where Lock on is useless and leaves you running blind while you fumble with the camera.

He's pretty mean when he's close to death but that's when I usually spam Ragnarok, even if I attack him while he's invincible, I am too so I don't take damage either. He's about patience and a bit of practice.