First boss battle in the Den of Behemoths.
Not a full strategy but a quick important point: Partway through the boss battle he will take whatever money you have on you and use it for health. I don't know if the boss had 500,000 health (as I had gotten that achievement before this boss battle) but it took me a good 10-15 minutes to beat him after he used all my gold for health. Either spend all your money before the boss or go for the 500,000G after this boss battle to make it significantly easier.
Also, if you do decide to go in with a bunch of gold you will get it back after the completing the fight.
The other strategy I would use is equip the Aegis Plate (which nullifies all damage from the spikes on the ceiling) and just Invert to the ceiling. You can avoid a good portion of his attacks up there, really only dodging the coin flips and his double-hand smash.