I dunno if there is already a guide, there's so many threads, but here's my whack at it (you can sticky it if you want, not that I want it to be stickied, and not that I don't want it to be stickied). Here's a pretty good guide to busting up Sephiroth in Kingdom Hearts II. First of all, to even acquire the ability to fight Sephiroth, you must go to the Dark Depths past the Bailey and approach him. There, a small scene will occur and then the battle begins. Now, on to the guide!
Recommended Levels-
Beginner: 90-100
Standard: 70-100
Proud: 60-100
Abilities and Items Useful in Battle-
Berserk Charge: An extremely useful skill that pumps up your combos and unleashes a neverending assault, taking away essential damage. Attack is changed to Berserk. The only flaw is that it breaks your finishing combos to dust, and when Sephiroth is at the peak of death and you are using this ability, it is almost impossible to kill him. A good trick is when he's almost down, use an aerial combo to finish him off.
Ultima Weapon: This weapon is not a must, but its very helpful. The ability it has is MP Charge, and it is good for when you need Curaga to bail you out.
Curaga: Like I said before, Curaga can bail you out. It's an essential tool that can be your primary source of healing instead of wasting away on items that can be used for Sin Harvest. Use Curaga as a full resort.
Elixir and Mega-Elixir: I kiss this items with love and joy. These two are incredibly awesome, as they pump you up on HP and MP full. However, these are very rare, and though you can synthesize them (look at a synthesizing guide for details), they should be used only as a last resort or for Sin Harvest. Equip your item pack with the whole rack of em, and if you don't have much, Hi-Potions and Ethers will do.
Boosts: Any Strength, Magic, and Defense Boosts you can use should be used on Sora immediately. The boosts will definitely play a crucial role in this battle. If you need more AP for Berserk Charge, use your AP Boosts.
Shortcuts: Use the Shortcut box in your time of need. This box can be accessed by pressing L1, and can be altered using the Customize tab in the Main Menu. Use the shortcuts as you wish; this is how I did it: Triangle- Elixir/Hi-Potion, Circle- MegaElixir/Ether, Square- Curaga, Ex- Curaga (Curaga is essential!!).
Aerial Recovery: This ability is good for when Sephy knocks you into the air and combos begin to entangle. Without it, you won't last long!
Once More: You need this for Sin Harvest. Nothing more to say.
Aerial Dodge: Master Form's (or Valor's?) primary ability. It is useful in some situations of the battle.
The Battle Itself - Into the Darkness
Okay, when you're ready, approach Sephiroth. You can skip the scene if you'd like. When it's over, immediately start jabbing the triangle button. As soon as the combo is over, start Berserk Charging away. Don't give him time between your combos; if you do, he'll teleport and use his air combos to strike most of your HP. As soon as your Berserk Charge runs out, he'll teleport and slash cut you into the air. As soon as you're up there, he'll pummel and pester you with air combos. As soon as you flinch in midair, press circle to use Aerial Recovery. Try to hit the ground and use Curaga as soon as possible. Remember, only use Curaga when you have 40% of your energy.
When he begins to summon bluish darkish orbs and his body starts to glow purple, just lock on to em and attack em. Remember to stay on the ground; you'll need to guard his flurry attack he performed at the beginning of the battle. Sephiroth will then summon a pillar of flames and will engulf you in them, heavily taking most of your HP. Just Jump and if you have Aerial Dodge, use it too. The pillars will magnet you close to them, so try to jump away as fast as possible. When you're far away from the pillars, it would be wise to use Curaga. Also, Elixirs (or some other items, like Hi-Potion) are acceptable to use if you run out of MP and you seriously need some healing.
Now, time for Sin Harvest. Sephiroth will jump into the sky and mutter something (the only thing audible is 'Heartless Angel'). Then, a halo appears on top of you and all your HP and MP is gone. Now's the time for use of the Elixirs. Make use of your Shortcuts box. When Sephiroth begins to shoot meteors at you, just jump and Aerial Dodge em until he stops. Then, close in for the kill. That's all the advice I can give ya! Hope I helped ya, and if I did; maybe a rep point wouldn't be much to ask?
And a bonus: here's an unlocking tournament's guide! Made by Bluejuan85.
Pain & Panic Paradox Cup (Level 60)
How to Unlock: Clear all Underworld Arena Cups
Cerberus Cup Paradox Cup (Level 70)
How to Unlock: With the exception of Final Form, Form levels should be at level 5 or higher. Also, Pain & Panic Pardox Cup must be unlocked.
Titan Cup Paradox Cup (Level 80)
How to Unlock: Summon level should be 5 or higher. Pain & Panic Paradox Cup and Cerberus Paradox Cup must be unlocked.
Hades Cup Paradox Cup (Level 99)
How to Unlock: Clear all previous Cups in the Paradox set, clear Space Paranoids episode 2, have max Form Levels, and Summon level should be at level 7.
Recommended Levels-
Beginner: 90-100
Standard: 70-100
Proud: 60-100
Abilities and Items Useful in Battle-
Berserk Charge: An extremely useful skill that pumps up your combos and unleashes a neverending assault, taking away essential damage. Attack is changed to Berserk. The only flaw is that it breaks your finishing combos to dust, and when Sephiroth is at the peak of death and you are using this ability, it is almost impossible to kill him. A good trick is when he's almost down, use an aerial combo to finish him off.
Ultima Weapon: This weapon is not a must, but its very helpful. The ability it has is MP Charge, and it is good for when you need Curaga to bail you out.
Curaga: Like I said before, Curaga can bail you out. It's an essential tool that can be your primary source of healing instead of wasting away on items that can be used for Sin Harvest. Use Curaga as a full resort.
Elixir and Mega-Elixir: I kiss this items with love and joy. These two are incredibly awesome, as they pump you up on HP and MP full. However, these are very rare, and though you can synthesize them (look at a synthesizing guide for details), they should be used only as a last resort or for Sin Harvest. Equip your item pack with the whole rack of em, and if you don't have much, Hi-Potions and Ethers will do.
Boosts: Any Strength, Magic, and Defense Boosts you can use should be used on Sora immediately. The boosts will definitely play a crucial role in this battle. If you need more AP for Berserk Charge, use your AP Boosts.
Shortcuts: Use the Shortcut box in your time of need. This box can be accessed by pressing L1, and can be altered using the Customize tab in the Main Menu. Use the shortcuts as you wish; this is how I did it: Triangle- Elixir/Hi-Potion, Circle- MegaElixir/Ether, Square- Curaga, Ex- Curaga (Curaga is essential!!).
Aerial Recovery: This ability is good for when Sephy knocks you into the air and combos begin to entangle. Without it, you won't last long!
Once More: You need this for Sin Harvest. Nothing more to say.
Aerial Dodge: Master Form's (or Valor's?) primary ability. It is useful in some situations of the battle.
The Battle Itself - Into the Darkness
Okay, when you're ready, approach Sephiroth. You can skip the scene if you'd like. When it's over, immediately start jabbing the triangle button. As soon as the combo is over, start Berserk Charging away. Don't give him time between your combos; if you do, he'll teleport and use his air combos to strike most of your HP. As soon as your Berserk Charge runs out, he'll teleport and slash cut you into the air. As soon as you're up there, he'll pummel and pester you with air combos. As soon as you flinch in midair, press circle to use Aerial Recovery. Try to hit the ground and use Curaga as soon as possible. Remember, only use Curaga when you have 40% of your energy.
When he begins to summon bluish darkish orbs and his body starts to glow purple, just lock on to em and attack em. Remember to stay on the ground; you'll need to guard his flurry attack he performed at the beginning of the battle. Sephiroth will then summon a pillar of flames and will engulf you in them, heavily taking most of your HP. Just Jump and if you have Aerial Dodge, use it too. The pillars will magnet you close to them, so try to jump away as fast as possible. When you're far away from the pillars, it would be wise to use Curaga. Also, Elixirs (or some other items, like Hi-Potion) are acceptable to use if you run out of MP and you seriously need some healing.
Now, time for Sin Harvest. Sephiroth will jump into the sky and mutter something (the only thing audible is 'Heartless Angel'). Then, a halo appears on top of you and all your HP and MP is gone. Now's the time for use of the Elixirs. Make use of your Shortcuts box. When Sephiroth begins to shoot meteors at you, just jump and Aerial Dodge em until he stops. Then, close in for the kill. That's all the advice I can give ya! Hope I helped ya, and if I did; maybe a rep point wouldn't be much to ask?
And a bonus: here's an unlocking tournament's guide! Made by Bluejuan85.
Pain & Panic Paradox Cup (Level 60)
How to Unlock: Clear all Underworld Arena Cups
Cerberus Cup Paradox Cup (Level 70)
How to Unlock: With the exception of Final Form, Form levels should be at level 5 or higher. Also, Pain & Panic Pardox Cup must be unlocked.
Titan Cup Paradox Cup (Level 80)
How to Unlock: Summon level should be 5 or higher. Pain & Panic Paradox Cup and Cerberus Paradox Cup must be unlocked.
Hades Cup Paradox Cup (Level 99)
How to Unlock: Clear all previous Cups in the Paradox set, clear Space Paranoids episode 2, have max Form Levels, and Summon level should be at level 7.