I just had a thought; maybe there's also the equivalent of 'Omega' of FF in the this Daikessen...
IMO it would go something like: you could access it after you beat the game and possibly beat all the bosses on the 4th difficulty (... I'm assuming once you finish the game on 'Muzukashii' or something, it unlocks the 'Oni' difficulty and you could switch to it by going back to that difficulty switching guy in the base), then in some far away corner of the world map or somewhere really really REALLY conspicuously hidden, that ultimate youkai would appear as an extra boss battle; song is Hataraku2000 on Oni (bahahaha), and maybe some HP bordering around to the point that even if you use the strongest drum, that drum will sound really annoying and you still need to get over 90%...
Just some daydreaming due to my experience with RPGs always having a SOAIGHEIUGBUEB boss
IMO it would go something like: you could access it after you beat the game and possibly beat all the bosses on the 4th difficulty (... I'm assuming once you finish the game on 'Muzukashii' or something, it unlocks the 'Oni' difficulty and you could switch to it by going back to that difficulty switching guy in the base), then in some far away corner of the world map or somewhere really really REALLY conspicuously hidden, that ultimate youkai would appear as an extra boss battle; song is Hataraku2000 on Oni (bahahaha), and maybe some HP bordering around to the point that even if you use the strongest drum, that drum will sound really annoying and you still need to get over 90%...
Just some daydreaming due to my experience with RPGs always having a SOAIGHEIUGBUEB boss