Ok, my guys were worst than your guys when I beat him. Debarrier is essential in defeating him, it negates Sephiroth's wall defense making him vulnerable again.

Make your weakest guy ( Cid ) " the healer ", and attach cure3 to your all materia. Heal when our guys get low hp ( not too low ). Attack with Cloud ( limit breaks are good ) normally. Vincent should attack normally and use the good enemy skills.

The most important things to remember are these:

1. Cast Debarrier on Sephiroth when he casts wall ( this is most important )

2. Use up your summons when wall is down ( especially bahamut )

3. Attack normally and use limit breaks when wall is down

4. And finally keep your guys healed with all + cure3 and items like elixers ( second most important ), heal while you wait for Debarrier to negate wall...

I hope those tips help, Safer Sephiroth isnt that hard, youll defeat him eventually. Just keep your guys healed and cast debarrier. Also plan your next move while his meteor attack ( the super long mathematical one ) video plays.