In the first phase, Sephiroth will walk around the arena, occasionally performing Rapid Mow Down, Stepping Mow Down, or Figa Wall. It is possible to hit him with Keyblade combos by jumping towards him to dodge his Rapid Mow Down, but it is more reliable to focus on Strike Raid or Sonic Blade. If Scan is equipped, Sephiroth's HP Gauge will not appear to deplete at first even though he is taking damage.
Once Sephiroth's first, red HP bar is depleted, he will enter the second phase, telegraphed by him holding out his hand and saying "Come." In the second phase, Sephiroth will run instead of walk, and more frequently use Rapid Mow Down. His first action will be to cast Heartless Angel, and he will occasionally recast throughout the rest of the phase. He will also begin using Wild Dance.
Once Sephiroth's second, yellow HP bar is depleted, he will enter the third phase, shrouding himself in a dark aura. His first action will be to cast Supernova. He will also begin using Shadow Flare.
Kingdom Hearts II
Sora should have his Defense as high as possible, through the use of equipment such as Ribbon and Cosmic Arts, or by applying Defense Boosts. Ultima Weapon and Circle of Life are useful Keyblades for their MP regeneration abilities, while Decisive Pumpkin is useful for its Combo Boost. For maximum mobility, equip Sora with High Jump, Aerial Dodge, Glide, and Slide Dash; for defense equip Guard, Counterguard, Aerial Recovery, Leaf Bracer, Second Chance, and Once More, and for offense equip MP Haste, MP Rage, and Trinity Limit. Defender, Combo Boost, Air Combo Boost, Finishing Plus, Retaliating Slash, and several Elixirs are also useful, but not as essential. Reflect and Cure should be set to shortcuts, preferably at their Reflega and Curaga levels, as well as any stocked Elixirs and, in Final Mix, Limit Form's Auto Limit.
Sephiroth always opens with Flash, so be prepared to use the "Block" Reaction Command, then use the opportunity to attack with Trinity Limit and Keyblade combos. If Sephiroth warps to use Cut Combo, be prepared to guard or use Reflect, then attack again. Sephiroth will not use the more difficult-to-block Aerial Cut Combo if Sora does not hit him with a combo finisher, so it is useful to forgo using finishers during the first phase unless proficient in using Reflect. Curaga is the best way to heal during the first phase, and can be followed with Berserk Charge to deal lots of damage without Sephiroth breaking free. However, if Sora uses Berserk Charge in the air, Sephiroth will teleport away and perform Flash after several strings of combos, and if Sora uses it on the ground, Sephiroth will still teleport away, and may counterattack if Sora fails to connect with any of his hits. If Sephiroth does Aerial Cut Combo, be sure to use Retaliating Slash to catch Sephiroth and deal an aerial combo.
After losing four and a half bars of his health bar, the battle will shift to its second phase as Sephiroth calls out, "Know your place."
In the second phase, Sephiroth will begin using Firaga Wall and Shadow Flare, often in conjunction with Flash, and his Cut Combo will have greater reach and speed. Reflect will deflect each of these attacks while damaging Sephiroth, and there is a small window of opportunity to attack Sephiroth directly as he finishes Firaga Wall.
Sephiroth will also start using his signature technique, Heartless Angel. As soon as he begins to say, "Descend... Heartless Angel!", ignore any other obstacles like Shadow Flare and use High Jump and Aerial Dodge or Trinity Limit to immediately close in on Sephiroth and hit him. If Sora fails to interrupt the technique, or is too far away to even try, immediately consume an Elixir to recover health, and prepare to deflect Sephiroth's followup attack.
After losing nine and a half bars of health, the battle will shift to its final phase as Sephiroth gains an aura of darkness and calls out, "All shall be one with me."
In the final phase, Sephiroth is much faster, and will fly around low to the ground instead of walking. The best time to heal is after Sephiroth performs one of his basic techniques. Sephiroth will also start using Moving Cut and Meteor, which can be dodged or countered.
If using Limit Form, be sure to use Limits frequently (especially Sonic Rave) to attack and heal, but unless proficient in using Guard, avoid using their final blows to prevent Sephiroth from retaliating. Due to Sephiroth's speed, Strike Raid's invincibility frames may not always occur when needed, so be careful to time the attacks well.
Notes and References