Well, what level are you? You can beat the game at level 50 without too much trouble if your materia is set up properly.
Here's what you need to do to make sure you can beat him around this level:
Make sure Barrier and MBarrier are maxed and have both of them combined with a maxed ALL (you want this going 24/7 along with Regen)
Make sure you have Cure 3 (or the best cure, can't remember which) combined with an ALL.
Make sure Regen is maxed and have it combined with an All (you want this going 24/7 along with Barrier/Mbarrier).
Make sure your limit breaks are full before heading into the fight (kill Jenova with Knights of the Round, save limits for Sephiroth).
Make sure you have plenty Ethers and Elixers, as you need full MP all the time throughout the fight.
Basics of figthing Sephiroth:
Slap on a Barrier or MBarrier (or both if he does melee and magic, I can't remember which he does), put on Regen, then start your summoning. If any of your characters run out of summons, use the highest DPS elemental magic you have (make sure it's equipped with All as Sephiroth has multiple hit zones in his first form).
I cannot remember if Knights of the Round resets after his first form. If it doesn't reset and you only have 1 star, you can only cast it once throughout the whole fight. I suggest using it on his second form if that's the case.
Use Bahamut ZERO as much as you can as it'll deal a lot of damage (second strongest summon in game).
Always, always always, keep up your Barrier/MBarrier and Regen. Never let them go away, which is why as I said earlier, make sure you have tons of Ethers and Elixers, to keep your MP up full.
The fight is really not too difficult once you do these steps and hit around level 50-60, which you should be roughly.
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