First you have to name yourself something with eight letters, and get a shiny nidoking to Lv100, then go to the st. anne, but let yourself get killed so the boat doesn't go away.
Then go back to the dock, and look for the parked truck. Inside the truck there is a half-eaten donut, take it.
Leave the docks, go to the cerulean gym and fight the second trainer, NOT the first one, repeat, NOT the first one. leave the gym and you'll find yourself in bill's secret garden. There will be a mew waiting there. Go there, talk to it and start the battle. In battle, use the half-eaten donut.
Mew will transform into hoopa and send you through black hole to the distortion world, where you'll need to find an azure flute.
Once you do it (it's behind one of the waterfalls going sideways) play it, and nothing will happen, of course, because you don't know how to play the flute.
Go back to where you started and hoopa will take you to viridian's trainer school, and talk to the teacher to learn how to play the azure flute. Play the flute.
Heavenly stairs will appear to lead you to the hall of origin, in front of arceus.
Battle arceus. In the middle of the battle, quote nietzche and dawkins et al, to conclusively prove its inexistance. Once you do that, arceus will disappear and you'll appear at the sinjoh ruins, where you'll find an egg.
When the egg hatches, it will be a missingno, you'll have to be very careful here, or your game will crash.
Go to the PC
Select missingno
Choose option "shoot this abomination straight in the noggin'"
missingno is now dead. You'll find 100x sacred ash in your backpack.
Take the sacred ash to eusine, he'll want it for his beauty treatments, and he'll trade them to you for an item called USE THIS TO EVOLVE CARBINK.
Take a carbink to the lighted room deep down in reflection cave, this carbink has to be, as you said, knowing flash, calm mind and safeguard. It also has to be Lv100 and have pokerus, but not cured.
At the center of the room, take one step back, three steps left, one step front, one step right
Hammertime
take two steps back and press A. Nothing will happen, of course, since the ground there is empty.
throw away USE THIS TO EVOLVE CARBINK, it won't help you.
Go back to the pokemon center, save carbink in the PC.
Get diancie beacuse you spent a coulpe of months doing stupid things to try and force carbink to evolve, when carbink is inmediately followed by goomy in the pokedex, therefore it doesn't have an evolution.
Now you have a diancie.
