So Demifiend. I did it on Casual as a test of the overall strategy and it worked well; I suspect I can also do it on Normal, but Hard would take a lot longer.
The issue is that the Demifiend is built to ensure no defensive strategy is foolproof: you can never stop their many Dekundas and Debilitates, they do seem to occasionally Freikugel you even if nobody has Null/Repel Phys, and you can't stop all the ailments or Chaotic Will even with Abyssal Masks for everyone. So on Hard you're always playing with, say, a 10% chance of a trainwreck every turn, and there's no way to kill him under, say, 20 turns (unless you do the Adversity trick). So that's what makes the fight interesting, of course, but I'm not sure I want to slog out the many reloads of a 30+min battle. Maybe in the future.
I definitely think they missed a huge trick on, well, none of the enemies having ANY AI except for Demifiend and a couple of other occasions. It's really weird that enemy boss Magatsuhi ends up becoming a free gift for the player because you can guarantee what they'll do on 90% of bosses and dampen it. Just changing that around - where all bosses "learn" a player's null/repels after one bounce, or even use Spyglasses on you - would completely change things.
Has anyone ever found all the Miman without walkthroughs? My best is 152 without looking anything up online.