The shiny variations of Pokémon in Pokémon Go are incredibly difficult to come by. It’s a little bit easier if you’re hunting down the ones you find at raids, so long as you’re willing to waste a raid ticket on the event. If you truly want some of the rarest shiny Pokémon, you’ll be hunting after the shadow shiny Pokémon. A shadow legendary Pokémon is rare, and only comes from Giovanni, but a shiny shadow is even rarer.
There’s only one method to acquire a shiny shadow Pokémon. You can do this by battling one of the three Team Rocket grunts or leaders and no, Giovanni is not included in this. You have to battle against Arlo, Sierra, and Cliff, or any of the grunts their Pokémon. They will rotate out their first Pokémon every few months, and that new Pokémon will the new potential shiny Pokémon that you can capture. You won’t know it’s a shiny Pokémon during the battle, so you have to battle them, win, and then try your luck at seeing if a shiny will arrive when you attempt to save it.
Grunts do have a chance for shiny Pokémon, and those are extraordinarily rare.
The Pokémon Arlo, Sierra, and Cliff usually rotate every so often, so you may need to do a little investigation to see what shadow Pokémon you can capture. The chance of you encountering a shiny type is always pretty low, with the potential hovering around 1 in every 450 encounters.