Sephiroth wasn't hard.
Kurt Zisa was harder than Sephiroth I think. He sealed your fucking magic and became immune to physical attacks after it was unsealed.
Anyway, Sephiroth just has a long sword, nothing else. Determine the range so that he can't hit you.
He's not dangerous at all in his first phase.
His second phase is. But if you have Strike Raid and have him locked on, Descend Heartless Angel isn't a problem. Omnislash is the fucking problem.
Then his third phase isn't a problem.
If you know his attacks and know how to dodge them, he's not a problem. I beat him about 25 times in a row because he was the fastest way to get to Level 100.
Kimi wa ne tashika ni ano toki watashi no soba ni ita
Itsudatte itsudatte itsudatte
Sugu yoko de waratteita
Nakushitemo torimodosu kimi wo
I will never leave you
