I don't know if anyone has already answered your question..but let me give you a few tips.
Depending on your mode of play, your stats should be somewhere between 65-99. Personally, being level 99 makes me feel more secure when fighting him. Have all drive forms at level 7. Strength and Defense should be 70-75. Equip Ultima Weapon, Trinity Limit, Second Chance, Once More, and all MP Haste abilities. Customize the shortcuts with Curaga, Reflect, Ethers, and Elixirs. Be sure to disable Auto-Load. Don't be stingy when it comes to synthesizing your Armor. It WILL help with your defense!
Fighting Sephiroth the first time, if you're not focused, expect to die quickly. Don't assume that you'll beat him the first few times. Learn his tactics well. When he begins to say "DESCEND HEARTLESS ANGEL," jump up, lock on to him, and attack before he can finish casting it. With Once More and Second Chance, this attack will drain your HP to 1, and your MP to 0. So be sure to hit him sometime before he says "ANGEL".
Pay attention to the Reaction Command "Block." Use it when it appears. If not executed, you will receive massive damage or die. Glide, Jump, or Quick-Run away when pillars of fire appear. If caught in the inferno, use Trinity Limit to temporarily make yourself invincible. Except for using Trinity Limit, do the same when he casts Meteor. The screen will darken for one second, that way you'll know this attack is coming. Every time he teleports, lock on to him or follow him as fast as you can. Knowing where he is constantly, will be a great advantage to you. When he summons dark balls of energy, use Reflect or simply hit them away. Coming into contact with them WILL deal some damage.
When he glows purple, he will be invincible for a short while. RUN AWAY!! Regroup, then unleash another barrage of Trinity Limit and Berserk, or your normal attacks.
Keep your HP and MP levels up, and try not to be intimidated by the 15 bars of life that he has. Although his attacks become more aggressive as you continue to demolish his health, don't freak out. It means his end is near! By merely trying to survive all his attacks while getting in quite a few of your own, you'll defeat him in record time.
These are just a few tips..I'm not saying that this is fool proof, but try it out anyway and see how it goes for you...if not, then I hope someone else can help you better than I can. Good luck!