at least lvl 75 or higher and ultima weapon, to have a better chance. high jump, quick run, air dodge, glide all at lest lvl 2. and have at least second chance, once more, airial recovery, explosion, and leaf bracer equipted.
you all should know to press triangle at start. after that he will usually pull off combos of at least five. to avoid that he usually stands at a spot and lets you come to him. run next to him and quickly use 2 quick runs after each other to avoid the hits. hopefully you have slide dash so you can hit him from a distance away and combo him. remember to pull of a another combo right when he leaps back you will have another chance to combo him, after the second one he will warp, if you get away from him after the first combo you do and don't get the second combo in, he will usually warp right behind you and hit you up into the air, and if he does, use air recovery immediatly, then keep pressing X to pull a combo off on him in midair which is a good stratagy if you don't mind taking a little damamge. after a bit you should get his pattern down and be able to counter him easily. ( he will also do charges towards you occasionally like at the start of the match, when you hear him say "now thats enough" press triangle immediatly because he will ambush you)
once you weaken him down, he will make dark spheres cirlcle you which is very annoying! but when he does it try to ambush him and pull of a combo so you activate explosion at the end. explosion will take care of the spheres. he will also use sin harvest which is simple to avoid. just remember to lock on to sephiroth every time he dissapears, if you do that, you will be able to catch him glowing in midair and chanting his sin harvest move. just jump and air dodge to reach him up high to pull off a combo and stump sin harvest. you must be fast though. if you don't stop him in time you want to use at least a high potion or elixer to heal and immediatly after you use it PRESS TRIANGLE or he will put your life down to where it was and you will have to heal again!
eventually he will power up and have a pink glow. he is obviously more difficult because his combos will hit you even if you quick run or glide, he uses meteor, and he runs around like crazy. just keep your distance and attack when he uses sin harvest or hits you up in the air (air recovery and attack quickly). or attack him when he runs around, but that can be difficult because he is QUCIK! meteor is an annoying attack, but to avoid it jump and glide left then to the right. repeat it until he is finished with the move. keep attack when the time is right, heal when needed and you should be able to take him down easily. (note: you can't hurt sephiroth when he is using meteor.)
hope this helps