So, I can't find any real info on this one. But to my knowledge, this fight should be very similar to the previous [U ] Sephiroth fight we did many months ago.

Multiplayer AI by /u/tfmurphy

BOSS: Sephiroth (VII)

Difficulties: 160 (Ultimate ) and 200 (Apocalypse)

Mastery Conditions:

• Win with 4 or more party members alive.
• Reduce Sephiroth's ATK.
• Reduce Sephiroth's DEF.

Moveset (credits to SolitaireD):

• Ultimate: Sword Flash - 2 AOE piercing physical (NAT)
• Octaslash - AOE physical (NAT)
• Fire Blossom - AOE Fire PHY
• Iai Strike - ST massive physical very low % KO (NAT)
• (not sure what this attack is) - ST fixed damage
• Sudden Cruelty - 2 random physical high % Stun (NAT)
• Shadow Flare - AOE massive physical (NAT)
• Ultimate: Meteor - AOE massive NE magic (NAT)
• Attack - ST PHY
• Counter Scintilla - % to counter all actions with ST PHY Self Haste

Overview:

• He doesn't have any elemental weaknesses or resistances.
• At specific HP thresholds, he will use Ultimate: Sword Flash, which deals 2x NAT physical attacks to all party members, that ignores DEF.
• He also counters all abilities with Counter Scintilla, which deals a single-hit PHY to one random party member, then buffs himself with Haste.
• When his wing appears (about 40% of remaining HP), Ultimate: Meteor becomes unlocked in his moveset. This is his only magical attack in his moveset throughout the whole fight.
• Among his other abilities includes Sudden Cruelty (2x random-target heavy PHY damage), Octaslash (AoE NAT physical), Fire Blossom (AoE Fire-elemental PHY physical), and Shadow Flare (AoE NAT ranged physical).

Tips:

• Tauntaliate deals with Counter Scintilla. In the event you don't run one, bring Banishing Strike/Dispel to deal with it. Keep in mind that Banishing Strike may trigger Counter Scintilla, but Dispel won't. Tauntaliate is still the easier option IMO, plus it negates Sudden Cruelty.
• Keeping in mind that Ultimate: Sword Flash ignores DEF, you might want to bring stacking ATK debuffs to mitigate its damage. Or Physical Blink.
• Magic Blink can be good for dodging Meteor, if you choose to skip Shellga.