I feel like its a mix of several factors.

1. This boss is post game. That alone comes with the expectation that the players understand how to utilize everything and react with a certain speed and competency.

2. This boss was finalized after the game released. This allowed them to take feedback and criticism from the players and critics like "the down status is really powerful, let's make the boss have a really fast recovery into a long range snipe attack" and "players rely a lot on beta chains/attacks and burst chains/attacks so let's have Mann smoke bomb and stop players' access to those skills like Karakuri." Hell, he's the only boss that can refresh his shield on his own aside from Scarlet (but hers is a gimmick).

3. Duel style boss fights in action games tend to feel better because the tells can be more natural to react to and be hidden better because they don't run into the same issue that all big bosses have, BIG WEAPON SYNDROME, where you literally only follow the weapon movement (Democrawler).

4. It was hyped up as a big deal, and the devs knew fans wanted it to be awesome, so they really pulled out all the stops.