Man, was that boss tough. My levels weren't as high as yours when I beat it, though, so it should be doable for you. I had to tweak my setup slightly after losing to it a couple times, and the time I won was an insane war of attrition--desperately hurling Olives of Life (L) to give my Saving Grace character 9999 HP, whereupon the enemy team would proceed to whittle that away with their multitude of attacks to my one. I ended up getting out of that cycle of destruction thanks to a lucky pull with the Patience move essentially giving everyone an extra turn all at once at one point, allowing me to get back on my feet and return blows with a vengeance. What a fight!
I highly recommend giving everyone the passive hunter ability Patience. And Saving Grace on as many characters as possible too for these uber-tough boss fights.
I'd recommend it so that you can play around with them a bit prior to the endgame. Keep in mind that their respective bosses are arguably harder than the Ch 4 bosses, so arrive prepared. But they're very rewarding to earn and--in a way--slightly trivialize many of the fights afterward.
@TriforceBun It's definitely doable, but it was a sort of a perfect storm of unfortunate circumstances that screwed up my tactics, and that's what's so frustrating since the battle's so long. I was basically using Tressa as a Runelord to spam her sidestep ability on the whole party, rendering them practically invulnerable, but then I got all my buffs wiped at the worst possible time, didnt't have time to build them back up before he went all out and killed everyone.
I like this game a lot, and the battle system is a lot of fun, but I don't think it's as interesting as in games like Etrian Odyssey III/IV or Xenoblade Chronicles, so we'll see how much patience I'll have with making minor tweaks here and there to beat the post-game stuff.
EDIT: I wonder how the random enemy encounters in this game actully work. They don't seem entirely random, or at least seem to have certain triggers. For instance, I went on travels between Sunshade and a certain cave a lot, and every time I went from the former to the latter I would have an enemy encounter in the exact same spot every time, even if I took a slightly different route getting there, walking a few extra or few less steps. Same thing when I went from Noblecourt to Forest of Purgation.