How's Ultimate?

[KageTokage]

It honestly doesn't look that complex mechanically but the DPS checks are extremely tight, and pretty much all of the mechanics will one-shot you or even the entire party if you mess up at them, making it easily the most unforgiving fight in the game.

I think the "all the mechanics in the game" thing was an extreme exaggeration, as you're only dealing with select mechanics from each of the bosses and, but it gets hairy when you start having to deal with them simultaneously and have almost no safe zones to work with due to how tiny the arena is.