This is a secret boss i recommend doing it at the end of the game when you have at least 700-800+ might. And use status effect damage boost artifacts to fight him.
Go behind the counter of the Freeman Tacos shop, hit him 3 times and the secret boss fight should start.
He is really hard to beat with 9999 health remember to use status effects like fire or bleeding to keep damaging him while you heal your characters between rounds.
He will summon a smaller version of him sometimes be sure to kill him as soon as possible.
Save in the taco restaurant before the fight because it's a hard one.