Safer Sephiroth
Your first priority should be setting up Wall and casting Regen and Haste on the party. Also, heal anyone who may have been seriously injured in the last battle and restore MP to those who may have exhausted their supply. Sephiroth has several powerful attacks. The Pale Horse does a small amount of damage and it hits the party with every status effect in the book. Be sure you have a Esuna or a Remedy handy after to counter this attack. The Shadow Flare hits a single character for nearly 7000 points of damage. Unless your chacters have high levels or you're packing a couple of HP-Plus Materia, this is something to worry about. There's not much you can do about the attack, but hope that Wall is there to absorb some of the damage. Deen is a fairly week attack that does about 1000 to 1500 points of damage, but in a battle like this, nothing can be taken for granted. Then there's Sephiroth's summon spell, the Super Nova. This lengthy summon engulfs the party in a blast of cosmic porportions and typically does about 3000 to 5000 points of damage. Be sure you have Cure3 ready after this one.
Keep Wall up throughout the battle and quickly remove any status effects Sephiroth might cast on the party. Use DeBarrier to remove Sephiroth's barrier and to strengthen your attacks. Always try to use attacks that hit more than one time like 2x Cut,Comet2....anything. If you can keep your characters alive and move quickly, Sephiroth shouldn't be too tough.