i have never finished SMT3, i defeated all of the hell sub-bosses and then got really bored of the game when i learned that automaticaly doing so, force you into a fight with the super secret boss of the game and the only way to deal any sort of damage to him is a skill that ignore physical resistance because the super secret final boss resist everything (though maybe i could have used megidola? i never bothered to check)
it wouldn't be the first rpg i play to 99% and then quit right at the ending anyway
the tip i give everyone, abuse stat changing move, most of the boss battle will quickly devolve into stat change tug of wars, two cast of fog breath make the difference between getting hit hard and the boss missing 90% of his attack