About Shredding Amnesieri...
I remember when I've beaten him it was on my first encounter at lv122 Kaguya had 10 MP MAG SPD gems and 200 MAG/SPD level.
Just loaded my 1.1x save after the last boss to fight him again.
Now trying lv122 without gems and every stats lv100 like everyone.
The more the battle last the faster he gets.
At some point he's so fast at 7500 ATB I lose to his 6xxx ATB (can't put safely a glass canon he will get a turn even if switched right after his turn, like 100xx vs 1000x) with -50% SPD DBF (thanks Hina... but I need more).
So it's a luck-based damage race or a trick lies undiscovered.
Also tried Rin with Cat's Walk, Extra attack and the SHK main-equip with Reisen on the front to reduce his ailments resistances.
He very rarely gets shocked, less than 1/10.
Didn't try anyone outside of Kaguya and Eiki yet.
Hope some can deal good damage to rush him and allow to take risks but from what I've read...
You still get the Stat Gems for doing it at level. And there the whole challenge of beating it at recommended. Victory feels hollow when you just super overlevel and smash through it with no difficulty.
Except Shredding and the two 20F Strengthen Bosses. They deserve it.
Not sure about the rush, though. I haven't beaten it yet.
The only Strengthen Bosses close to him would be the Glowing Azure Giant well seeing how our teams are different I can't really judge.
The Strengthen Boss Rush is not hard, but tedious.
The hardest bosses are the 2 wasps because you can't afford risks, I had to make a semi tank/damage dealer.
Once I defeated them, everything has gone as planned, the battle took me 1 hour (retries not included ;_;).
I'm missing 2 items that afaik are only from boss drops, The Black Goddess of Fertility, and one of the 20F guards. Mirror i think it was. Can i get these drops when fighting the V2 battles?
There's no unique sub-equip in the page 5 except the last one you can't miss, you will find them later in some chests can't tell you where though.