Sephy also has a set attack pattern. This might help, so here it is:

1. Wall
2. Shadow Flare
3. Physical Attack
4. *Flies/Ascends higher* (This is not an attack, but it uses up one of his turns)
5. Pale Horse
6. Super Nova
7. Break
8. *Descends lower* (Again, it's the same as #4)
9. DeSpell
10. Deen
11. Physical Attack
12. *Ascends Higher* (Same as #4 and #8)
13. Pale Horse
14. Super Nova

After #14, he will repeat 7-14 until he dies.

From GameFAQs: Final Fantasy VII (PS) FAQ/Walkthrough by Arctic