You see, I know a lot of people who had trouble beating Sephiroth in KH2, but I beat him while being level 45 just by blocking, dodging, and healing at just the right time. Granted, I tried memorizing him telegraphing certain attacks, and that overall is what helped me beat him, and every time since. KH1, still haven't beat him there ._.
Anyway, I would say when I was young I could never beat Sephiroth at the end of FF7. Honestly I've never beat FF7, and only know the ending by Youtube. I've tried replaying FF7 a few times since to finally beat it and get bored with it by disc 3. He was the first boss I ever fought in a video game that had multiple forms, and I completely didn't expect it.