Sephiroth becomes around 5x more difficult at level 99. But then at that time, you're expected to have KotR, Omnislash, etc, so it sorta balances out.

Oh, if you want to make it more difficult, you can do one simple thing: don't equip ribbons.
Also, don't use KotR, Level 4 Limits etc. Or you could just not use materia altogether. Or just go fight the Weapons.