Well, I know how to defeat these guys in ultra moon. But I'm using Necrozma to defeat the Pokemon champion of Alola as well as Giovonni. Because he has strong moves and his powers are very strong, you know.
Just use pokemon that has moves that are stronger than the one you are fighting, check the pokemon weakness on serebii, go for the 2x or 4x damage output weakness if they have a 4x weakness. Anything work in-game. If you are under level, then level up.