What's with all the cowards!? Never run from a battle unless you've got a character confused that has flash as his attack command. Look, ff7 is one of the easiest rpgs you'll play, it holds your hand for 99% of the game.
Saviour Sephiroth (the one winged angel) can be tough if you are unprepared, but he has between 80,000 and 400,000 hp depending on a number of events. I'm going to assume you do not have Knights of the Round Materia, so at the most, 320,000 (if you use KOR on the last Jenova, he gets an 80,000 HP boost). Additionally, each character you have at level 99 adds 30,000. Which I will also assume you have 0. So that's an additional 240,000 (aeris doesn't count). So you are sitting at 80,000 hp. Not very much at all.
Now let's work this out. Right off the top he will cast "wall". Use dispel magic and it will remove it, you may have to do this a few times.
As you know, he can hit for heavy damage, so hopefully you have a few elixers or megalixers to use. At the very least, x potions, which you can buy. Try to organize the order of your items so that the ones you need to use are at the top of the list, also, before the 3 battles set the ATB gauge to wait (which is done through the menus).
He will go in the air at various points in the battle, have long range materia on your heaviest hitter. S-mine item will hit for over 1,000 damage. You can buy a piece of equipment called a wizard bracelet, which has 8 joined materia slots.
I assume you have low hp, did you know that hp plus materias can stack? Equip a few so that your hp are well over 5k, optimally, 9999.
Try to save up your limit breaks from the bizarro sephiroth battle, I know that battle can be a pain in the ass, but you've beat him once before right? Shouldn't be a problem, especially with two parties.
Pale horse is likely gonna kick your ass a few times, I certainly hope you have some remedy's in stock, else you will have to use up a materia slot to use transform magic on your party members. It sucks, but pretty much your only option.
There's not much you can do about "supernova", this is what you need the megalixers for, that or have cure3-all, preferably mastered. The good thing is that supernova won't kill you, just kick your ass. It has a chance of causing various status ailments, so a rememdy is your top priority in that situation, followed by healing up. W-item materia would really help here, but I doubt you have it.
Break will likely hit for 6k, pheonix downs are your friend. His regular attack will hit for around 2k, no big deal. Just try to save up limit breaks, and go in with your strongest party (obviously), and save your mana for healing if you run out of megalixers and elixers. Equip long range on who you'd like to do the most attacking and don't bother doing buffs on yourself as he will dispell and don't bother trying to do status effects to him as he is immune. Treat it like that big blue scorpion in mt. Nibel. Play cautious, prepare to deal with status effects and taking big damage, attack when you are confident he won't kill you and heal when you are the slightest bit worried. Use your healing items liberally as you won't need them after this fight, any elixers and megalixers you've picked up on your journey should be used after his big shots and remember, change the active time battle (ATB) to wait, so you can take your time to think.
Try not to let status effect stack up, else you'll go nowhere and fast. Remember, he's only got 80,000 hp, which is really a cakewalk, even without ultima weapons you should be hitting for about 3,000 per attack. You can do the math, slow and steady is the pace, I just hope you didn't squander your megalixers, as they will make things much easier for you.
If this doesn't help, well, the only answer is grind it out, get stronger, and at least get Ultima Weapon (as was previously advised). Good luck
I was pretty proud.
