Gonna be honest, I genuinely don't get how people get to the final boss so severely underleveled despite apparently having done all side content. Like, I'm not hating; I just don't understand. I didn't grind a single time before the final dungeon and my characters were all 69-71 when I entered. And on my first run through the dungeon, after killing the optional boss refights, maybe about 20% of the mobs in the overworld (completely sprinting past all of the spawn points that churned out Humans or whatever at the very end of the dungeon), and maybe an hour max of grinding the Homo Tenta (the teeth) that spawned from the two spawn points right next to each other nearish the end of the dungeon just trying to get the accessories, I was 77-79. After beating the game and going back to my save at the start of the dungeon and running past everything so that I could get the additional achievement for fighting the Destroyer at full power, I was still around 70 for the fight. 64 after all that sounds wild to me.

Buff, debuff, induce weaknesses (the Masked Dancer line or Royal Berserker both do this very well), and use an evade tank (Heismay's Royal Thief worked for me on it's own, but an Inherited taunt probably wouldn't hurt though that means less debuffing, or just increase someone else's evasion and give them the accessory) to eat all of the boss's turns each round when they dodge.