The Legend of Heroes: Trails into Reverie: Coda Final Boss (Abyss +100) - Solo Estelle

Difficulty: Abyss (+100 added to enemy level)
Restriction: Only allowed to use Estelle Bright in battle, everyone else must be KO'd immediately

Level: 250
HP: 7531240
BRK: 7154678
STR: 8437
DEF: 4875
ATS: 7687
ADF: 4375
SPD: 681

Moveset:
Regular attack
Dispersal Thorns: Stabbing attack that inflicts Deathblow
Demolition Light: Fires a large beam of light that delays targets
Gives itself a physical reflect and heals 5% of its max HP
Sprinkles particles around an area
Sweeps a laser at targets standing close by
Photonic Rest: Slams the ground, pushing targets away
Charges a craft that reduces HP of all targets to 1
Succumb to Fate: Charges up for instant all-vanish next turn (only used at less than 10% of max HP left)
Metempsychosis: Attack that it tends to do after recovering from break (Up to 2 times it seems, can only be avoided via magic evasion)
Wedges of Fate (Anti-Order): 10.0x delay to opponent turns (3 turns)

This boss is the result of someone on the boss design team waking up and choosing violence (7.5 million HP and nearly 700 SPD). Without a specific strategy, this boss is practically impossible. The most popular strategies are constant usage of Brave Orders that AT Advance the party, or 100% evasion tanks. But even then, the all-vanish attack activating at 10% or less HP means you pretty much need to time the moment you break the boss with the moment the boss' HP reaches that threshold and then go ham with every strongest move you have, otherwise it is an instant game over. The anti-order can allow it to get up to 20 turns in a row even if your characters have 300 SPD, so use Chrono Burst or any move that has 0 delay to end the order early (The boss can use the anti-order up to 3 times). The perfect guard is a guaranteed OHKO against whoever dares attack it while it is up (unless they have a craft guard and/or if an all-cancel move is used).