I killed Sephiroth. o_O; At Level 75.
After about 10 times of dieing--I learned his entire moveset and really brought it to him. =/ He's a pansy.
Here's some tips. ^_^
-Be Level 75+ (Most guides say 80--but I think 75 is just fine)
-Ethers/Mega-ethers, and Elixers of any kind help tremendously! You'll need to have them, and Cura/Curaga Customized to the L1 shortcut menu.
-Abilities needed are:
Upper Slash And/or Finisher Leap (Help keep combos goin)
Retaliating Slash (Needed to stop "Omnislash" or "Tele-slash"
Trinity Limit (Needed to stop DHA or "Descend Holy Angel")
Glide (Level doesnt matter--Helps dodge his Fire-wall)
Combination Boost (Helps keep Trinity Limit longer)
Leaf Bracer (Helps you heal easier)
Item Boost (Helps you not waste items)
MP Rage (Not needed, but help)
MP Haste (Not needed, but help)
Defender (Not needed, but help)
Second Chance and Once More (Keep you from dieing at all his combos and attacks)
And you'll of course need your trivial stuff, like Combo plusses and scan--etc. Slapshot, and all those other Action abilities would be nice, too.
-Warning for ya': He's got 15 life bars. <_<;
-The beginning of the battle is allways the same--just hit Triangle. He does this many other times in the battle--just remember one important thing: YOU CAN ONLY BLOCK THIS IF YOU'RE ON THE GROUND! Otherwise it doesn't work..
After every "Block", you have a free shot to get in a combo or two--try to knock him into the air with Upper Slash or Finisher Leap to buy a little time (And some nice aerial damage)
-Try to stay away from him at all times--his regular attack swipes do a lot of damage.
-Make sure you have Retaliating Slash--Wait for him to teleport at you and begin his "Omnislash", once you're in the air wait for him to appear again--Once he appears again, press Square and then go back to mashing combos with X. When he falls to the ground you probably have enough time to get another combo in.
---Repeat the above until about 5 of his life bars are gone---
-Ok, now he begins to summon dark orbs around you--you can take the time to hit them away, or just ignore them--they dont really matter. What really matters is not letting him run at you and begin raping your HP bar with that insanely huge sword. o_O; So just avoid him, and repeat what you were doing.
-Now that his life has 1/3 of it gone, he'll start using Fire-wall and Decend Holy Angel (Which I'll refer to as DHA for the rest of the post).
--Fire wall is easy to avoid--just jump and use Glide to not be pulled into it.
--When he rises into the air--he should say something along the lines of "Decend (unintelligible) angel" [I think he says "One-winged", but most everybody says that it's "holy". *Shrugs* I dont care..] Go to Limits, and use Trinity to instantly warp up to him and beat the crap out of him. Trinity should knock him down about 2 bars and a half. After Trinity is done--you should be able to get another combo out of him.
-----Once he's down to about 5 bars of life-----
-He'll summon up the last of his power and truly become a "Berserker" Sephiroth.
-Watch out for Meteor--he raises into the air like he's gonna do DHA, but if you over react an select Trinity, you'll just be wasting your MP and hitting an Invicible Sephiroth.
-Just wait for the right moment--and try to hit Trinity to finish him. It's the easiest way to get a hit in with the insane way he's running.
-----If you actually beat him with my guide, Grats!-----
And I hope that helps. ^_^