Decided to do a little writeup of this guy to give a clearer picture on what's the best way to tackle him.

Fallen One

Level: 85

HP: 40000

Attack Type: Cut

STR: 88

VIT: 76

AGI: 69

LUC: 65

TEC: 82

Damage Resistances:

Cut: 100% Bash: 100% Stab: 100%

Fire: 100% Ice: 100% Volt: 100%

Ailment Resistances:

Instant Death: 1% (Attempting to get around this with Ailment Boost will result in that chance being multiplied by 6%)

Petrification: 1% (Attempting to get around this with Ailment Boost will result in that chance being multiplied by 6%)

Sleep: 20%

Panic: 10%

Poison: 30%

Blind: 30%

Curse: 20%

Paralyze: 20%

Stun: 20%

Head: 75%

Arm: 10%

Leg: 10%

Skills:

Darkness Curse: Does not use a body part. +30 Speed. Purges any buffs that are on the party, and is guaranteed to inflict Head bind, Arm bind, and Leg bind. Pierces immunities. Also binds the Fallen One. The binds placed on it depends on when it casts this skill. See below for more details.

Dead Claw: Uses the Arms. +/-0 Speed. 30 Hit Rate. A STR based melee Cut attack that randomly attacks the party 3 to 6 times, and deals 130% per hit. Has a 10% chance of inflicting Instant Death.

Bloody Blade: Uses the Arms. +/-0 Speed. 99 Hit Rate. A STR based melee Cut attack that deals 200% damage to the entire party.

Infernal Flight: Uses the Legs. -3 Speed. 99 Hit Rate. A STR based ranged Fire attack that deals 250% damage to a row, and has a 20% chance to inflict Panic.

Ice Pellets: Uses the Legs. -3 Speed. 99 Hit Rate. A STR based ranged Ice attack that deals 250% damage to a row, and has a 20% chance to inflict Sleep.

Gale Volt: Uses the Legs. -3 Speed. 99 Hit Rate. A STR based ranged Volt attack that deals 250% damage to a row, and has a 20% chance to inflict Curse.

Supernova: Uses the Head. -50 Speed. 200 Hit Rate. A TEC based ranged Almighty attack that deals 400% damage to the entire party. Used every 5 turns.

The Fallen One can be considered EO4's equivalent of the Elder Dragon fight in EO3. Like with the Elder Dragon, the Accumulative Resistance does not apply to the Fallen One's bind resistances. Meaning that you can inflict binds as much as you want, and using Releasal Spell to reset his bind resistances is a waste of time. However, one infliction of any ailment such as Poison will render him immune to those, so Releasal Spell must be used to undo that.

Darkness Curse is the central gimmick for this fight. The Fallen One only casts it when his HP reaches certain thresholds. And while it always fully binds your party and purges their buffs every time he casts it, it only binds specific body parts depending on when he casts it. The body parts that get bound during the fight are listed below.

Start of the battle: Head and Legs.

76%: Arms and Legs.

51%: Head.

26%: Arms.