... I did it, somehow.
I don't know how, and while this hunt was not as long as my hunts for shiny Charmander and shiny Groudon, it felt quite long because I was not a huge fan of button-mashing through Gladion's speech for 30 seconds each time. It took around 3,200+ SRs. The IVs are "Outstanding potential!" with perfect ATK, DEF, and SPD, while the SP. ATK and SP. DEF have "Pretty good" ratings and HP has a "Very good" rating. I'll let the pictures speak for themselves.
I've gotta say, good luck to Echo Nacyl. I hope you get your shiny Null soon.
The golden coloration is reminiscent of shiny Arceus. That cannot be a coincidence