Sephiroth's fight can be essentialy divided into 3 phases. Each phase lasts 2 of Sephiroth's HP bars (the first is invisible). The abilities you MUST have equipped are two MP Rages, MP Haste, ONE combo Plus (Sephiroth teleports after 4 hits), Guard, eventually Arcanum and last but not least Dodge Roll. A few Elixirs are nice just to be on the safe side but you probably won't be needing them with this startegy.
In the first phase Sephiroth uses a quick slash, a jumping sword sweep and summons three pillars of fire. If you want to just retreat for a while you'll notice an attack pattern that keeps repeating which consists of a slash, a jump towards sora, a quick slash, one or two jumping sword slashes, another quick slash then summon the fire. Your best bet is to Dodge Roll those sweeping attacks TOWARDS Sephiroth then get your four-hit combo in as soon as he gets back. He will always teleport behind you and immediately use a jumping sword sweep which you can dodge roll under and start hitting using your four hit combo again. This should get you through the first phase relatively easily although he will sometimes hit you no matter how good you get at dodge rolling. The simple solution is to cast Aero and/or Cure often when you feel you're in peril.
In the second phase Sephiroth uses all the above and two new moves: Heartless Angel (yes I know it sounds like Sin Harvest but it ISN'T, the name of the technique can be found at the official Final Mix site) and Omnislash (the succession of fast blows with a great finisher). He will always use Heartless Angel after Omnislash so beware. Sephiroth will announce this phase by taunting Sora ("Come On!"). The first notable difference will be that he runs and will take the initiative rarely giving you time to heal. He will start by running towards you but instead of taking him head on like a moron retreat to one side of the arena. He will teleport himself behind Sora once he gets close but on the other side of the arena, meaning that if you're badly positionned you will be skewered by the infamous Heartless Angel. If you did it well, he will teleport just next to you and give you ages to interrupt the lethal attack. Hit him four times then he will teleport behind sora and begin the same series of attacks as in the first phase but frequently inserting a Heartless Angel and less often an Omnislash. If you want to be on the safe side, try to stay on the middle of the arena so that no matter which end of the arena Sephiroth teleports to do his attack you will be within dodge rolling distance to interrupt him.Omnislash is another attack you want to look out for which he announces by saying "Come Power!" or "Prepare yourself!". He will only actually hit sora every 3 swipes so if you keep count and block the corresponding strikes you should get out of this attack unhurt. If you don't want to bother timing it right the best you can do is keep healing while he does it (hence the vital need of MP Rage). After the finisher (Sephiroth hovers in the air then strikes down, just block after he says "No more games") he will teleport at the other end of the arena to do Heartless Angel so be ready to dodge roll towards him to prevent an untimely death (yours in this case). This should go for 2 HP bars then when you begin to see the green one he will power up and summon an aura (a very nastly one at that).
Phase 3 introduces a whole new array of devastating attacks attacks as well as powering up the previous ones. The first thing worth ntoing is the abundance of dark balls of energz Sephiroth will create by doing anything from running to the basic sword slashes. In a way it means his weapons have a longer reach than ever. He will also call these dark balls to circle and close in on Sora which you can dodge roll or jump out of. If you just ignore the damage (Aero is vital here) you can get 3 hits in which will be just enough to make him teleport behind you and do it again. This is also your best opportunity to use Arcanum. Sephiroth always starts phase 3 by summoning a circle of dark balls to close in on sora. Once he does that, he will proceed to summon Meteor Shower. This attack is really annoying when on the lower levels because it will hit xou at least once at a rate of 99%. When he summons the meteors you should Glide or keep jumping to avoid the large shockwaves . The meteors will then proceed to circle Sephiroth (he is immune to all attack just like in Omnislash) either in random directions, in which case you will definitely be hit at least twice, or anti-clockwise in which case you should run and dodge roll in the same direction as them to avoid being hit. After a few seconds of this the meteors explode then sephiroth sends down that great sphere of energy he had been gathering all along. You can avoid damage from this one by dodge rolling BEHIND Sephiroth AFTER he throws the thing (you have around 2 seconds to do so.). Now all hell breaks loose. You will notice rather quickly that Sephiroth has a new sword slash which makes him spin like a cyclone that is fast, powerful and hard to dodge (he drives you crazy by saying "Dodge This!!", the bastard). The tempo of the attacks are what what make this battle teeth grinding. He will jump around, use a sword swipe, then teleport (or not) behind Sora going Dodge This all the time. Aero is a REALLY vital necessity here. You might also find yourself swept away in this frenzy of attacks then suddenly realise that he's on the other side of the arena charging the ever-feared Heartless Angel. Try your best to make sure the you're within dodge rolling (or super gliding if you have it) distance to ANY point of the arena within seconds because if not, you're dead. Speaking of dead, Sephiroth still uses Omnislash, more powerful than ever, but the real bummer is that he will frequently use two of them consecutively. As a result, you barely have time to recover from the first before he goes and does it again, which is in turn, of course, followed by a Heartless Angel attack. After using Omnislash, Sephiroth will summon the dark balls again to close in on sora then teleport into the middle of the arena to do Meteor Shower. Thus the circle is complete and the whole third phase mostly consists of Sora healing himself and getting a few combos in when he can. You might want to make frequent use of Arcanum when you can afford it (which will often be the case given all those MP you gain thanks to MP Rage) as it makes you invulnerable (but not to Heartless Angel I've discovered) and gets a decent load of hits in. Of course, if you have enough strength you can just go Arcanum crazy and spare a few elixirs/ethers to afford doing the combo until death (his, with any luck) parts you from Sephiroth.
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Well I try basicly everything ppl try to give meh but it never works!