After defeating the Piokemon League, you can return to the Ruins of Conflict in order to battle Tapu Koko, the Island Guardian of Melemele Island. Tapu Koko is a very fast Pokemon, with offensive strengths that are no laughing matter. It's ability also creates an Electric Terrain, boosting all of its Electric Type Attacks and preventing sleeping.
Unfortunately, Tapu Koko is part Electric Type, which means it isn't possible to paralyze it. You also won't be able to put it asleep while there is an Electric Terrain, which basically means you'll have to catch Tapu Koko without inducing any type of Status Condition. And since it's at Level 60, this means it's going to be a tough capture.
However, outside of its Nature's Madness move that only cuts your Pokemon's HP in half, Tapu Koko only has Electric Type Attacks. If you have a few Ground Types on your team, this will definitely play to your advantage, as it will never be able to defeat it. Also having a Pokemon or two with with the move False Swipe will help, since it will ensure Tapu Koko is left with at least one HP. If you have the Rotom Power that makes it easier to catch a Wild Pokemon, then you should definitely consider using it here as well.
After lowering Tapu Koko's HP as much as you can and feel comfortable with, Ultra Balls are the way to go. They give you the best chance at capturing Tapu Koko, so make sure you have plenty on them. And don't forget to save before facing Tapu Koko, just in case you accidentally knock it out and don't particularly want to face the Elite Four again before being able to find it once more.