As your trainer is getting set to leave Ula'ula Island with both Hau and Gladion in hopes of finding Lillie at Aether Paradise, Kahuna Nanu will appear on the Malie City Docks and challenge you to your Ula'ula Island Grand Island. Because of the suddenness of it all, most players might not be ready to face him with his strong Dark Type Pokemon. In order to make sure this doesn't happen to you, definitely try to be prepared beforehand.
All of Nanu's Pokemon will be at or above Level 43, so make sure you're Pokemon are near the same or even higher so you can go toe to toe with this Kahuna. If you were smart enough to do the trade for Hawlucha back in the Route 2 Pokemon Center, it will again come in handy against Krokorok and Persian, annihilating both with its speed and Super Effective Fighting Type Attacks. If you were wise enough to make such a trade, all you will have to deal with is Sableye, a Pokemon not really known for offensive prowess. If you didn't make such a trade, Fairy Type Pokemon do best against Nanu, as all three of his Pokemon are weak to such Pokemon and Attacks.