Here's a detailed guide for defeating Sephiroth Reborn in "Final Fantasy 7 Rebirth," including requirements, a walkthrough, strategies, and additional tips:
Requirements:

Sephiroth Reborn Boss Guide in Final Fantasy 7 Rebirth
•Location: Fought at the end of Chapter 14 in the Edge of Creation.
•Rewards: Leads directly into the final battle with Sephiroth, crucial for story progression.
•Party Composition: Multiple phases with different characters:
•Phase 1: Cloud solo
•Phase 2: Tifa, Barret, Cait Sith
•Phase 3: Zack
•Phase 4: Red XIII, Barret, Yuffie
•Final Phase: Cloud and Aerith
Materia:
•Fire Materia: Useful in various phases due to Sephiroth's lack of elemental weaknesses.
•Healing Materia: Essential for all phases, especially for survival in solo or duo fights.
•Barrier Materia: To reduce damage from Sephiroth's powerful attacks.
•Assess Materia: To understand his attack patterns and any resistances.
Gear:
•Equip Cloud with the Hardedge for damage or Twin Stinger for Counterstance; Tifa with Mythril Claws for stagger; Zack with his Buster Sword; Red XIII with Mythril Collar; Yuffie with 4-Point Shuriken; Barret with EKG Cannon; Aerith with Guard Stick for magic and healing.
•Gear should focus on high defense and resistance to magic damage.
•Level: Aim for around level 55-60 for a smoother fight, but strategic play can compensate for lower levels.
Detailed Walkthrough: 
Phase 1 (Cloud Solo):

Sephiroth Reborn Boss Guide in Final Fantasy 7 Rebirth
1.Attacks: Whisper Vortex, Errant Whisper, Maelstrom, Hell's Gate, Interred Horror
2.Strategy:
3.Focus on Body: Attack the body of Sephiroth Reborn to pressure him. Use Braver or Focused Thrust.
4.Dodge: Constantly dodge the attacks, especially Maelstrom which pushes you back.
5.Climb: After damaging the body, climb Sephiroth's form to reach the head and deal damage.
Phase 2 (Tifa, Barret, Cait Sith - Bahamut Arisen Whisper):

Sephiroth Reborn Boss Guide in Final Fantasy 7 Rebirth
1.Attacks: Flame Breath, Thruster Spin, Umbral Bombardment, Gigaflare
2.Strategy:
3.Dragon First: Focus on defeating Bahamut Arisen Whisper, using Synergy Abilities and elemental spells.
4.Wings: While fighting Bahamut, destroy Sephiroth's wings to prevent healing of the summon.
Phase 3 (Zack Solo):

Sephiroth Reborn Boss Guide in Final Fantasy 7 Rebirth
1.Attacks: Lacryma Deus, Retribution, Interred Horror, Apocalyptic Fire, Zanshin
2.Strategy:
3.Chest: Damage the chest to pressure Sephiroth Reborn. Use Meteor Shots and Synergy Abilities.
4.Dodge: Stay alert for Retribution and Apocalyptic Fire, dodging these when possible.
Phase 4 (Red XIII, Barret, Yuffie):

Sephiroth Reborn Boss Guide in Final Fantasy 7 Rebirth
1.Attacks: Blessing of Calamity, Spirit of Calamity, Swarm
2.Strategy:
3.Core: The core is protected by a barrier; destroy the wings with Yuffie's Ninjutsu to expose it.
4.Elemental Match: Use the opposite element of what the wings are infused with for quick destruction.
Final Phase (Cloud and Aerith):

Sephiroth Reborn Boss Guide in Final Fantasy 7 Rebirth
1.Attacks: Apocalyptic Fire, Hell's Gate, Heartless Angel (reduces HP to 1)
2.Strategy:
3.Healing: Keep Aerith's healing ready for after Heartless Angel.
4.Focus: Hit Sephiroth with your strongest abilities, using Cloud's Punisher Mode for counters and Aerith for magic and support.
5.Limit Breaks: Use these when Sephiroth is staggered or low on health for maximum damage.
Strategies: 

Sephiroth Reborn Boss Guide in Final Fantasy 7 Rebirth
•Elemental Attacks: While Sephiroth Reborn isn't weak to elements, using Fire can still deal good damage.
•Stagger Management: Pressure by hitting weak points or during specific attack patterns. Cloud’s Focused Thrust, Tifa’s combos, and Zack's charged attacks help stagger.
•Defensive Play: Use Barrier or Manawall to survive long enough to heal post-Heartless Angel.
•Synergy Abilities: Use these in multi-character phases to deal with both the boss and its summons.
Tips: 

Sephiroth Reborn Boss Guide in Final Fantasy 7 Rebirth
•ATB Management: Conserve ATB for abilities in solo phases, use them for spells or healing in duo or trio phases.
•MP Management: On Hard Mode, use MP Absorption or be very selective with spells.
•Positioning: Always be on the move to dodge area attacks or to position for optimal attacks.
•Limit Breaks: Save for when Sephiroth is staggered or in the final phase for a powerful finish.
•Hard Mode: Equip Fire Materia with Elemental on weapons, use Barrier and Manawall strategically.
•Summoning: If possible, summon during the final phase for a significant impact.
•Character Synergy: Each character's unique abilities should be used to complement the others, especially in multi-character phases.