The highest level trainer Pokémon are found in the Pokémon League (Elite Four + challengers). The highest level wild Pokémon are found in Poni Gauntlet, right before the Battle Tree. Getting to Lv. 100 is quite difficult in Sun/Moon, which seems counterintuitive when you consider the shiny new Hyper Training, (Alola doesn't have Blissey Bases or O-Powers to farm Exp) but with enough patience it's possible. Going to Poni Gauntlet and chaining Pelipper and Bewear, combined with a Lucky Egg and two hearts in Refresh, will probably be the fastest way to grind until your Silvally can beat the Elite Four.