How do you beat Sephiroth in Kingdom Hearts 2?

To make this battle much easier, you should:
  • Be at least Lv70, but you can easily get to Lv90 by defeating all the powerful Nobodies in Twilight Town or the World that Never Was repeatedly.
  • max out your Drive levels, although you don't have to; you only need them to be high enough so you can use their Growth abilities, which will totally help you:
  1. You can easily level up Valor Form (gains 1 Exp for each hit you land on an enemy) just by using it wherever you go where there's lots of high HP enemies, so you don't defeat them too quickly to level up Valor Form.
  2. For Wisdom Form (1 Exp for each Heartless Sora defeats), You can defeat many Heartless at Pride Rock's Peak in Pride Lands where there's numerous Rapid Thrusters, at Fragment Crossing in The World that Never Was where there's many Shadows, and in Radiant Garden where there's many other Heartless along Castle Gate, Bailey, and Ravine Trail.
  3. For Master Form (1 Exp for each small Drive Orb and 3 Exp for each large Drive Orb), go to Candy Cane Lane, and run around to summon Heartless, and RC (Reaction Command) to spin the merry-go-round to force Drive Orbs out of the Heartless, and pick up all the Drive Orbs. When the Drive Gauge almost depletes, return to Santa's House to enter the World Map via the Save point while still in Master Form, then return to Santa's house to have a refilled Drive Gauge.
  4. For Final Form (1 Exp for each Nobody defeated), go from the World Map to Twilight's View, then go to Naught's Skyway where there's lots of Creeper Nobodies to defeat.
  • equip a Decisive Pumpkin, Fatal Crest, or Ultima Weapon
  • fill up your Item slots with Elixirs only, and equip Elixir as one of your shortcuts
  • The only magic you'll need are Reflega and Curaga, so equip them for Shortcuts, so you should only be needing 3 Shortcut commands total.
  • equip other equipment that increase defense and Dark resistance
  • equip the following abilities:

    1. Upper Slash
    2. Retaliating Slash
    3. Slapshot
    4. Sliding Dash
    5. Guard Break
    6. Explosion
    7. Aerial Sweep
    8. Aerial Finish
    9. 2 Combo Plus
    10. 2 Air Combo Plus
    11. Combo Boost
    12. Air Combo Boost
    13. Finishing Plus
    14. Berserk Charge (optional)
    15. Leaf Bracer
    16. 2 MP Rage
    17. MP Haste
    18. Defender
    19. Second Chance
    20. Once More

Now, on with the tactics.

As soon as the battle starts, use the Reaction Command right away. Then, immediately pummel Sephiroth with ground combos and then press Square to knock Sephiroth into the air to restart your combo but this time in aerial form. Sephiroth won't teleport away unless you miss or your combo completely stops (you land a finishing combo), so as long as you're in a combo with Sephiroth, you can continue to deal damage while he deals with the pain. After your Aerial Finish and Finishing Plus abilities take place, Sephiroth will teleport and try one of his own combos. Use a Reflega combo, then Glide around the map to avoid Sephiroth. After his combo misses, he'll wait and teleport closer to you to try again. Jump, and Aerial Dodge if necessary, then Reflega or Glide away again, but you should probably keep Aerial Dodge until your Gliding is close to the ground and for some reason you need to quickly switch direction, then you can let go of Glide to use Aerial Dodge then Glide again.

If Sephiroth uses Stigma (large fire pillars), stand next to him and cast a Reflega combo to reflect immense damage back at Sephiroth. If you're in MP Charge, just keep away and heal if necessary.

When Sephiroth is attacking, keep away from him at all times until he says, "That's enough." Try to be relatively close to the ground before he says that, so when he does, be ready to RC; you can't RC in the air, which would mean you need Curaga/Elixir instantly if you can't return to the ground soon enough. After you successfully RC, pummel him with a long ground combo with Explosion and Guard Break to deplete much of his HP (or knock him in the air with the Square button to hit him even more).

He'll teleport again, so keep away as usual. A benefit to having used Curaga recently is that you get to utilize Berserk Charge. Wait until you use RC again, and if you're still in MP Charge, hit him repeatedly in the air until he teleports, then avoid him again.

About Berserk Charge, the ability only activates when you're in MP Charge (if you've used Reflega to defend yourself too much or recently cast Curaga). So if you can get close enough to hit Sephiroth or if he says, "That's enough," so you can RC, then you can start pummeling Sephiroth (ground or midair, doesn't matter) with Berserk. He'll just keep getting hit until eventually he'll teleport away, whether or not your MP Gauges has been refilled or your Berserk combo breaks. When he does teleport away, he may try a combo on you or cast one of his spells, so Lock-on him when he reappears and cast Reflega if you can or simply Quick Run/Glide away.

If he floats in the air and says, "Descend, Heartless Angel," try to double jump to him and hit him right away to prevent his spell cast, then continue with a long aerial combo and finish with an Aerial Finish and Finishing Plus, yet eradicating even more of his HP. If you're too far away from Sephiroth when he starts to cast the spell that will deplete all your HP and MP, stand far away, and be ready to use an Elixir. Hopefully you haven't run out of those, or else you better be able to cast Curaga right away. Right when he floats back down, he'll always say, "That's enough." Obviously, this is another opportunity again to RC then pummel him on the ground then press Square to reset your combo to deal even more damage or to simply end with Explosion-Guard Break, assuming that you were able to heal quickly enough before RC.

When Sephiroth summons Shadow Flares (dark orbs) around you, ignore them; just hit them away, because they're probably only meant to distract you from his actual attacks. This occurs when he has about less than half of health left.

When he casts Super Nova, which launches meteors at you, just double jump really high then glide around until it stops. If you get hit, try to use Aerial Recovery to continue dodging. If you're hit on the ground, Quick Run far away, double jump, then Glide again.

If you equipped a Decisive Pumpkin (obtained from Halloween Town), you can easily remove almost 2 of Sephiroth's HP bars if you can drag your combo out long enough. Ultima Weapon (obtained from Synthesis) is more of to avoid missing and if you intend to use Reflega and Curaga a lot. If you don't have Ultima Weapon, at least you can use Fatal Crest (obtained after winning the Goddess of Fate Cup, unlocked after defeating Xemnas at the pinnacle of The Castle the Never Was) instead.

If Sephiroth manages to hit you with one of his combos, don't heal if you have more than 1 HP after he's done with one combo on you, so you still have MP to use Reflega if he casts Stigma (major disadvantage to him and a major advantage to you, since Reflega backfires Sephiroth's Stigma and he can't do anythign about it until he's done casting Stigma). Only heal if you have 1 HP, so you don't have to MP Charge so soon so you can preserve your emergency Elixirs.

Don't equip Negative Combo, because the point of the Upper Slash, Combo Pluses, Air Combo Pluses, Combo Boost, and Air Combo Boost (and maybe the Decisive Pumpkin Keyblade) is to easily wipe out a few of Sephiroth's HP bars in one combo before he teleports, since then you'd have to wait until you can RC again, because you'd want to keep your HP full throughout the first half of the battle and to avoid having to use Curaga at all until way later in the battle.

Don't equip Item Boost, because that's no point for Elixirs, unless if you equip a Hi-Potion as a fourth Shortcut command. Also, remember to Lock-on to him at all times. Once he teleports, you need to know where he is immediately.
If Sephiroth hits you and knocks you into the air, wait several milliseconds (yes, if he knocks you into the air and disappears, wait right when he reappears when he tries to start a midair combo on you; don't Retaliate Slash before he reappears, or you'll be hitting the air and leaving yourself wide-open when he does reappear to pummel you) before using Retaliating Slash to take an advantage of a long Aerial Combo against a will-be helpless Sephiroth.

Just keep up this strategy and you might be able to beat Sephiroth without ever losing an HP. By the way, you should probably save before you fight Sephiroth, and then after you beat him, save in another slot file, since you can never fight Sephiroth again after beating him once and in case if you just ever feel like fighting him again in the future, unlike in KH1.