Inspired by SMT V's Demi-Fiend fast kill, and decided to take a crack at it here in Digital Devil Saga. It won't win any Speed Record, but at least the party only has to endure one Gaea Rage.
Timestamp:
0:00 ~ Character Stats
All party members are Stunned before entering this fight to avoid Javelin Rain's Mute effect. This will require all five party members to pull off.
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0:18 ~ Girimekhala & Cu Chulainn
Since Demi-Fiend's party moves first, getting past the first turn without Girimekhala dropping someone with Mamudoon and without one too many critical hits from both Cu Chulaiin and Demi-Fiend were the biggest hurdles to overcome. Thankfully after that Cu Chulainn's Taunt script begins, the fight will become a bit more manageable.
Best case scenario would be the party has more than 500HP and Cu Chulainn did not start Taunting the party after Demi-Fiend's turn ends, if not take a gamble and use Black Sun. Most Magic combos in Digital Devil Saga has 100% accuracy, so they are not affected by Stun's secondary effects. Charge up everyone, heal only if everyone on the field have less than 500HP after Dekunda. Wait until a turn where Demi-Fiend's turn cycle ends with Cu Chulainn's Taunt. This ensures Ragnarok is buffed from Taunt's +2 Physical attack buff.
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2:22 ~ Pixie
Dormina is not an issue if it hits Serph or Heat, as the party will only need two Press-Turns to kill off Pixie before Demi-Fiend uses Gaea Rage. If Dormina hits 2 or more party members, or it hits Gale then the party cannot inflict enough damage to kill off Pixie before Demi-Fiend Gaea Rages. 2 Ragnarok are enough here, as a third Ragnarok will trigger Mediarahan before Arahabaki and Titania can be taken out.
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3:19 ~ Arahabaki & Titania
Transform Gale and revive Serph with Heat, take advantage of Arahabaki's weakness to Ice to bring his HP down to roughly Titania's and use the half turns to bring out Cielo and Argilla. Depending on how aggressive the Arahabaki's AI in casting Ragnarok , Debilitate, or Gate of Hell, use Black Sun, if everyone has more than 600HP and with no debuff placed other than speed debuff, else use Power Charged Ragnarok to accomplish the same goal. Have all party member Charged and Focused, find an opening where the Arahabaki and Titania does not place any debuff on the party take both of them out at once.
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7:57 ~ Mediarahan
With no demon on the field, Demi-Fiend is at less than or equal to half HP, he will full heal himself, then summon Parvati. Take her out before she uses Dormina, and Demi-Fiend will summon Cu-Chulainn instead of Gaea Rage.
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8:46 ~ Cu Chulainn & Girimekhala
Two more Teradyne, one with Ragnarok 's Defense debuff, and one more Power Charged + Taunt boosted Ragnarok are needed, before taking out Girimehkala and Cu-Chulainn again. Charge up everyone, hopefully everyone gets Muted from Javelin Rain and Demi-Fiend's turn ends with Cu-Chulainn's Taunt. Use Rancid Gravy to help with this process.
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17:11 ~ Pixie
Try to ensure everyone is Muted, if not at least Cielo should be, since if Dormina landed with Cielo, this will result in Gaea Rage and a Game Over. This is the final set of damage needed, the Demi-Fiend goes down with his Pixie waifu.
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