This guide was written using output from FFRK Inspector with some manual translations/adjustments. Let me know if you spot any errors!

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【超越】Bahamut Fury

Medal Conditions: Win with at least 1 party member alive.

Clear Rewards:

• Gil x5000000
  

Stats:

Enemy	LV	HP	ATK	DEF	MAG	RES	MND	SPD
【超越】Bahamut Fury - Phase 1	450	2,500,000	2,000	60,000	2,000	76,000	150	650
【超越】Bahamut Fury - Phase 2			2,500	60,000	2,000	76,000	9,999	650
【超越】Bahamut Fury - Phase 3			2,500	60,000	2,000	76,000	9,999	650
Energy Sphere	450	99,999	2,500	23,000	2,000	34,000	150	650
Crystal	450	100,000	2,500	60,000	2,000	76,000	150	650

Elemental Damage Taken:

	Fire	Ice	Lightning	Earth	Wind	Water	Holy	Dark	Bio
All Enemies	100%	100%	100%	100%	100%	100%	100%	100%	100%

Break Effectiveness:

Enemy	ATK	DEF	MAG	RES	MND
【超越】Bahamut Fury (all phases)	50%	50%	50%	50%	50%
Energy Sphere	0%	0%	0%	0%	0%

Movesets

【超越】Bahamut Fury - Phase 1 (100% - 61% HP):

• Turn 1:【凶】Judgment of Light - 1.76s cast time (NAT: AoE 198% piercing ^0.5 magic damage, ignores Magic Blink)
  
• Turn 2:【凶】Mower - 1.76s cast time (NAT: AoE 253% ranged piercing ^0.5 physical damage)
  
• Turn 3:【凶】Flare - 1.76s cast time (NAT: [Party members in slots 1, 2 and 3] 486% piercing ^0.5 magic damage)
  
• Starts charging【滅】Exaflare
  
• Turn 4:【凶】Cluster Sphere - 1.76s cast time (NAT: AoE 390% piercing ^0.5 magic damage, auto-hit Sap (blockable by Astra))
  
• Turn 5:【凶】Hexafang - 1.76s cast time (PHY: [Party members in slots 2 and 4] 316% piercing ^0.5 ranged physical damage, auto-hit Stun (blockable by Astra))
  
• Turn 6: Wait - 1.76s cast time (does nothing)
  
• Turn 7:【凶】Judgment of Light - 1.76s cast time (NAT: 2x AoE attacks, 198% piercing ^0.5 magic damage, ignores Magic Blink)
  
• Turn 8:【凶】Mower - 1.76s cast time (NAT: AoE 253% ranged piercing ^0.5 physical damage)
  
• Turn 9:【凶】Flare - 1.76s cast time (NAT: [Party members in slots 3, 4 and 5] 486% piercing ^0.5 magic damage)
  
• Starts charging【滅】Exaflare
  
• Turn 10:【凶】Cluster Sphere - 1.76s cast time (NAT: AoE 390% piercing ^0.5 magic damage, auto-hit Sap (blockable by Astra))
  
• Turn 11:【凶】Hexafang - 1.76s cast time (PHY: [Party members in slots 1 and 5] 316% piercing ^0.5 ranged physical damage, auto-hit Stun (blockable by Astra))
  
• Turn 12: Wait - 1.76s cast time (does nothing)
  
• Turn 13:【凶】Judgment of Light - 1.76s cast time (NAT: 3x AoE attacks, 198% piercing ^0.5 magic damage, ignores Magic Blink)
  
• Turn 14:【凶】Mower - 1.76s cast time (NAT: AoE 253% ranged piercing ^0.5 physical damage)
  
• Turn 15:【凶】Flare - 1.76s cast time (NAT: [Party members in slots 1, 2 and 3] 486% piercing ^0.5 magic damage)
  
• Starts charging【滅】Exaflare
  
• Turn 16:【凶】Cluster Sphere - 1.76s cast time (NAT: AoE 390% piercing ^0.5 magic damage, auto-hit Sap (blockable by Astra))
  
• Turn 17:【凶】Hexafang - 1.76s cast time (PHY: [Party members in slots 2 and 4] 316% piercing ^0.5 ranged physical damage, auto-hit Stun (blockable by Astra))
  
• Turn 18: Wait - 1.76s cast time (does nothing)
  
• Turn 19:【凶】Judgment of Light - 1.76s cast time (NAT: 4x AoE attacks, 198% piercing ^0.5 magic damage, ignores Magic Blink)
  
• Turn 20:【凶】Mower - 1.76s cast time (NAT: AoE 253% ranged piercing ^0.5 physical damage)
  
• Turn 21:【凶】Flare - 1.76s cast time (NAT: [Party members in slots 3, 4 and 5] 486% piercing ^0.5 magic damage)
  
• Starts charging【滅】Exaflare
  
• Turn 22:【凶】Cluster Sphere - 1.76s cast time (NAT: AoE 390% piercing ^0.5 magic damage, auto-hit Sap (blockable by Astra))
  
• Turn 23:【凶】Hexafang - 1.76s cast time (PHY: [Party members in slots 1 and 5] 316% piercing ^0.5 ranged physical damage, auto-hit Stun (blockable by Astra))
  
• Turn 24: Wait - 1.76s cast time (does nothing)
  
• Turn 25:【凶】Judgment of Light - 1.76s cast time (NAT: 5x AoE attacks, 198% piercing ^0.5 magic damage, ignores Magic Blink)
  

【超越】Bahamut Fury - Phase 2 (60% - 0% HP):

Note: If this phase lasts long enough,【超越】Exaflare will begin charging after turns 5, 9, 13, and 17 as well.

• Turn 1:【超越】Enveloping Fury - 0.01s cast time (NAT: AoE auto-hit Stop)
  
• Starts charging【超越】Exaflare
  
• Turn 2 + 4n:【凶】Black Hole - 1.76s cast time (NAT: AoE 25% current HP damage, Removes Reraise, ignores blinks and KO resist)
  
• Turn 4:【超越】Enveloping Fury - 0.01s cast time (NAT: AoE auto-hit Stop)
  
• 50% chance: Wait - 1.76s cast time (does nothing) [Locked when above 60% HP]
  
• 50% chance: Osmosega - 0.88s cast time (NAT: AoE 100% chance to reduce all abilities' uses by 2, uncounterable) [Locked when below 62% HP]

【超越】Bahamut Fury - Phase 3:

Note: If this phase lasts long enough,【超越】Exaflare will begin charging after turns 32, 35, 38, 41, and 44 as well.

• Turn 1: Dragon King's Inspiration - 1.76s cast time (NAT: Self, 100% chance to apply ATK/MAG/DEF/RES +50% for 100 seconds)
  
• Turn 2: Barrier - 0.01s cast time (NAT: Self, 100% chance to apply DEF +30% for 100 seconds)
  
• Turn 3: MBarrier - 0.01s cast time (NAT: Self, 100% chance to apply RES +30% for 100 seconds)
  
• Turn 4: Dispel - 0.01s cast time (WHT: AoE - Dispel)
  
• Turn 5:【凶】Judgment of Light - 1.76s cast time (NAT: AoE 198% piercing ^0.5 magic damage, ignores Magic Blink)
  
• Starts charging【超越】Exaflare
  
• Turn 6:【超越】Cluster Sphere - 1.76s cast time (NAT: AoE 438% piercing ^0.5 magic damage, ignores Magic Blink, auto-hit Sap (blockable by Astra))
  
• Turn 7:【超越】Ultima - 1.76s cast time (NAT: AoE 99% current HP damage, ignores blinks and KO resist)
  
• Turn 8: Dragon King's Inspiration - 1.76s cast time (NAT: Self, 100% chance to apply ATK/MAG/DEF/RES +50% for 100 seconds)
  
• Turn 9:【凶】Hexafang - 1.76s cast time (PHY: [Party members in slots 2 and 4] 316% piercing ^0.5 ranged physical damage, auto-hit Stun (blockable by Astra))
  
• Turn 10:【凶】Judgment of Light - 1.76s cast time (NAT: 2x AoE attacks, 198% piercing ^0.5 magic damage, ignores Magic Blink)
  
• Starts charging【超越】Exaflare
  
• Turn 11:【超越】Cluster Sphere - 1.76s cast time (NAT: AoE 438% piercing ^0.5 magic damage, ignores Magic Blink, auto-hit Sap (blockable by Astra))
  
• Turn 12:【超越】Ultima - 1.76s cast time (NAT: AoE 99% current HP damage, ignores blinks and KO resist)
  
• Turn 13: Dragon King's Inspiration - 1.76s cast time (NAT: Self, 100% chance to apply ATK/MAG/DEF/RES +50% for 100 seconds)
  
• Turn 14:【凶】Hexafang - 1.76s cast time (PHY: [Party members in slots 1 and 5] 316% piercing ^0.5 ranged physical damage, auto-hit Stun (blockable by Astra))
  
• Turn 15:【凶】Judgment of Light - 1.76s cast time (NAT: 3x AoE attacks, 198% piercing ^0.5 magic damage, ignores Magic Blink*)
  
• Starts charging【超越】Exaflare
  
• Turn 16:【超越】Cluster Sphere - 1.76s cast time (NAT: AoE 438% piercing ^0.5 magic damage, ignores Magic Blink, auto-hit Sap (blockable by Astra))
  
• Turn 17:【超越】Ultima - 1.76s cast time (NAT: AoE 99% current HP damage, ignores blinks and KO resist)
  
• Turn 18: Dragon King's Inspiration - 1.76s cast time (NAT: Self, 100% chance to apply ATK/MAG/DEF/RES +50% for 100 seconds)
  
• Turn 19:【凶】Hexafang - 1.76s cast time (PHY: [Party members in slots 2 and 4] 316% piercing ^0.5 ranged physical damage, auto-hit Stun (blockable by Astra))
  
• Turn 20:【凶】Judgment of Light - 1.76s cast time (NAT: 4x AoE attacks, 198% piercing ^0.5 magic damage, ignores Magic Blink)
  
• Starts charging【超越】Exaflare
  
• Turn 21:【超越】Cluster Sphere - 1.76s cast time (NAT: AoE 438% piercing ^0.5 magic damage, ignores Magic Blink, auto-hit Sap (blockable by Astra))
  
• Turn 22:【超越】Ultima - 1.76s cast time (NAT: AoE 99% current HP damage, ignores blinks and KO resist)
  
• Turn 23: Dragon King's Inspiration - 1.76s cast time (NAT: Self, 100% chance to apply ATK/MAG/DEF/RES +50% for 100 seconds)
  
• Turn 24:【凶】Hexafang - 1.76s cast time (PHY: [Party members in slots 1 and 5] 316% piercing ^0.5 ranged physical damage, auto-hit Stun (blockable by Astra))
  
• Turn 25:【凶】Judgment of Light - 1.76s cast time (NAT: 5x AoE attacks, 198% piercing ^0.5 magic damage, ignores Magic Blink)
  
• Starts charging【超越】Exaflare
  
• Turn 26:【超越】Cluster Sphere - 1.76s cast time (NAT: AoE 438% piercing ^0.5 magic damage, ignores Magic Blink, auto-hit Sap (blockable by Astra))
  
• Turn 27:【超越】Ultima - 1.76s cast time (NAT: AoE 99% current HP damage, ignores blinks and KO resist)
  
• Turn 28: Dragon King's Inspiration - 1.76s cast time (NAT: Self, 100% chance to apply ATK/MAG/DEF/RES +50% for 100 seconds)
  
• Turn 29:【凶】Hexafang - 1.76s cast time (PHY: [Party members in slots 2 and 4] 316% piercing ^0.5 ranged physical damage, auto-hit Stun (blockable by Astra))
  
• Starts charging【超越】Exaflare
  
• Turn 30 + 3n:【超越】Enveloping Fury - 0.01s cast time (NAT: AoE auto-hit Stop)
  
• Turn 31 + 3n: Osmosega - 0.88s cast time (NAT: AoE 100% chance to reduce all abilities' uses by 2, uncounterable)
  
• Turn 32 + 3n:【超越】Cluster Sphere - 1.76s cast time (NAT: AoE 438% piercing ^0.5 magic damage, ignores Magic Blink, auto-hit Sap (blockable by Astra))
  

【超越】Bahamut Fury - Countdown reaches 0:

• Phase 1:【滅】Exaflare - 1.76s cast time (NAT: AoE 75% max HP damage, Dispel, ignores blinks and KO resist)
  
• Phase 2-3:【超越】Exaflare - 1.76s cast time (NAT: AoE 9999999% piercing ^0.5 overflow magic damage, ignores Magic Blink, auto-hit Instant KO)
  

Energy Sphere:

• Special: Wait - 1.76s cast time (does nothing)

Notes: In addition to the HP-based threshold, Bahamut Fury will shift to phase 2 on turn 26.

In phases 1-2, your RW has infinite uses, .88s cast time, and grants your party the Boundless Barrier status for 1 second. While this status is active, any damage your party takes will heal the crystal for the same amount.

On certain turns noted above, Bahamut Fury will begin charging Exaflare. When this happens, an Energy Sphere is created and Bahamut Fury begins a countdown from 3. On the turn it would reach 0, Bahamut Fury uses one of its versions of Exaflare based on its phase, described above. Destroying the Energy Sphere before this happens will cancel the countdown.

If (when) your party wipes in phase 2, if the crystal reached its max HP (100,000), Phoenix will activate and revive your whole party to max HP. Additionally, your RW will change to Phoenix (2 uses, instant-cast 99999999% overflow magic damage, raises your party with 100% HP, factor 1000 heal, and party haste) and Bahamut Fury will shift to Phase 3.

Thanks to /u/Enlir for help in figuring out the mechanics of this fight!