Here's an easy method to grind in Pokemon Sun & Moon.
In a step by step process:
1. Obtain a Lucky Egg (Boosts overall EXP, like the Amulet Coin.)
2. Give the Lucky Egg to your strongest Pokemon
3. Activate the EXP. Share
4. Grind in the following areas; Poni Meadow, Poni Gauntlet, Pokemon League.
The result should be that EXP gained with the Lucky Egg and distributed by the EXP. Share should be boosted by a considerable amount. It's also recommended that you S.O.S. Grind, in tandem with Shiny Chaining if you want to, for optimal results.