Just found a bug where it looks like that you can play the last 2 levels on the "Simple" difficulty.
I'm streaming at the moment to figure out more but here it's how i did it twice:

Go in a level where you can choose the simple difficulty and leave it. Now go to the King Dice or Devil and just enter it with the "Regular" difficulty without pressing left or right on the difficulties. DONE!

I just 1 shooted the glassed and 2 shooted the chips with a fully loaded glass. The bunny has 2 cards in it but dont spawn.

Good luck with it!

So I found a little bit more:

King Dice:
1st - 1 full Charge = dead
2nd - 2 full Charge = dead
3rd - 2 full Charge = dead
4th - spawns under floor, cant hit the stones
5th - doesnt spawn
6th - doesnt spawn
so i cant reach the last 3 bosses and the final phase

Devil:
dies directly after phase 1

A bit more:

King Dice:
1st - 1 projectile is enough for each glass
4th - the super work on the boss to kill him, unfortunately non weapon works
7th - dies after few shots, doesnt move at all
8th - dies after few shots, doesnt move at all
9th - doesnt spawn
last phase - he went directly in the card attack and stick there without spawning any, unfortunately you cant hit him at all even with super art 3

King Dice:
- none of the mini-bosses moves

4th - you need 4 super attacks to kill him
7th - 2 full charge = dead (just for the numbers), chips go instantly if pink form after you parried those
8th - 2 full charge = dead

As a proof I just want to show a video where I used the tactic in a run:

Since "Frayzlo" was in the chat and helped, i think the hole run would be an "Simple - All Bosses" Run.

Interesting, when I finished the game for the first time and started to play some bosses on Expert mode, I remember that when I started a New Game, the difficulty of the first boss was selected to Expert and the selection was there on a invisible Expert button, because there is no expert mode when you start a new game.

Looks like the game always save the difficulty selection, and for the last bosses, it messes everything up.

Good catch.

I believe we're considering this a valid thing for Any. All Bosses needs to be on a consistent difficulty though

Glad to hear that the glitch might get into a run. I would be into that.

You can actually defeat the dominoes without Super Art 3 + Coffee if you aim down-right and shoot (I think it only works with spread, but feel welcome to try other weapons as well, could work with pea shooter) so if there's a way found to defeat king dice this could actually be viable for any% considering you don't have to go out of your way to get Super Art 3 and Coffee

how am I supposed to pass the King Dice if most of the mini bosses just don't spawn?

At the moment the is now way to do so. In the run I did the King Dice on regular.