The Ultimate boss's overdrive did like 9k damage to my team. If you don't have any ATK down debuffs up at that point, you die.

Ragnarok was fairly straightforward. As previously advised, don't put up any DEF down debuffs and hold your damage skills + Black Propaganda until after the first overdrive. That gives you 4 turns to push the boss out of Rage. My Burst wasn't ready so I facetanked the Raging Overdrive. With -65% ATK down it wasn't too dangerous, but the debuff hit one of my kamihime and I didn't have a cleanse ready, so she died while the boss was stunned. The boss was dead before the next overdrive.

Bring lots of attack down and anyone with a decent team should be able to clear it. I used Mordred to guarantee debuffs landed, but Joan with Black Propaganda would also work.