After you complete the Ula'ula Grand Trial, you can go up Mount Lanakila to challenge the Pokémon League.
Go to Tapu Village and go north from there to Mount Lanakila. Go north to the platform and you will have to battle Pokémon Trainer Gladion. He has a level 53 Crobat, a level 53 Zoroark disguised as Lucario, a level 55 Lucario, and a level 55 Silvally (Fire type if you chose Rowlet, Water type if you chose Litten, and Grass type if you chose Popplio). You get 8800 Pokédollars for winning.
Afterward, you get a Max Elixir.
Get on the elevator and press the button in the middle to go up.
At the top, to the right of the grass, there is an Escape Rope.
Go north and enter the cave.
To the north, you can battle Ace Trainer Seth. He has a level 51 Scyther and a level 52 Malamar. You get 3536 Pokédollars for winning.
Go west from the Ace Trainer and go up the ramp. In this area, you can battle Ace Trainer Kailee. She has a level 51 Shiinotic and a level 52 Clefable. You get 3536 Pokédollars for winning.
Go north from there and you can find a hidden Rare Candy at the end of a narrow path.
Go back to where the path splits, and go east past a rock. Follow the path and you will battle Veteran Alonsa. She has a level 52 Lickitung and a level 52 Goodra. You get 4576 Pokédollars for winning.
Go southeast from Alonsa and exit the cave to find TM84 Poison Jab.
Go back into the cave, then go north, and keep going west when the path splits. Have Machamp push the lower-left boulder to the left. Then go to the left and push the lower-left boulder down to create a shortcut to the exit.
To the north, you can battle Worker Ovid. He has a level 50 Relicanth. You get 2000 Pokédollars for winning.
Go east from Ovid and have Machamp push the boulder to the east. Then go north to find a Max Elixir, and go west to find a Max Revive.
Go back to the Worker. To his left, there is a ramp. Go up, and go outside.
There is some grass to the north. There is a hidden Full Restore on a rock surrounded by grass.
Go north and you can have a double battle with Sparring Partners Allon and Eimar. Eimar has a level 51 Bewear, and Allon has a level 51 Mienfoo and a level 51 Machamp. You get 3264 Pokédollars for winning.
Go north from there and you can have a double battle with Sparring Partners Craig and Jason. They have a level 51 Hawlucha, a level 51 Crabominable, and a level 51 Pangoro. You get 3264 Pokédollars for winning.
Go north into the cave. On the other side, go north and you will find Necrozma. Check on it. If you share the light of your Z-Power Ring with it, it will battle you, and you can try to catch it. If there is a hail storm in effect during the battle, Necrozma will lose health on each turn. Even though it is a Legendary Pokémon, it is easy to catch. If the battle started between 8:00PM and 3:59AM, just throw a Dusk Ball for a guaranteed catch. Otherwise, try throwing Ultra Balls.
Necrozma is holding an Ultranecrozium Z.
After the battle, Colress gives you an N-Solarizer and an N-Lunarizer.
Detour to Mahalo Trail
Now that you have Necrozma, you can optionally go to Mahalo Trail, which is north of Iki Town on Melemele Island, and go north to find Lillie near the Ruins. Talk to her and she asks you to battle and catch Nebby. Talk to Nebby to start the battle.
You can throw a Quick Ball at the start of the battle for about a 40% chance of catching Nebby. You can also inflict Sleep and throw Ultra Balls for a 40% chance if Nebby is still at full HP.
If you catch Nebby, you get Solganium Z if you are playing Ultra Sun and Lunalium Z if you are playing Ultra Moon.
Now that the bridge is fixed, you can go north. To the right of the north end of the bridge, you find an Electric Seed.
Back on Mount Lanakila
When you are ready, go back to Mount Lanakila. Ride the elevator up, go into the cave, go west at the split, then go north across the boulder that Machamp pushed earlier to create a shortcut. Keep going north to go outside, and go north again to return to the crater where you found Necrozma. Go down into the crater, then go east up a slope. The path will split, with a narrow path going north. Go north up that narrow path, and follow the path to find an X Sp. Def. Then go back to where the path split, and go east to enter another cave.
In this cave, you find an Icy Rock. If you level up an Eevee in this cave, it will evolve into Glaceon.
You can battle Backpacker Peren near the Icy Rock. She has a level 51 Emolga. You get 1224 Pokédollars for winning.
If you go south from the Icy Rock, there is a narrow path. When the path splits, go south, then at the next split go west to find TM13 Ice Beam.
Go east from there to battle Collector Minty. He has a level 51 Florges. You get 1632 Pokédollars for winning.
Go south from there and you will eventually go outside. There, you can battle Hiker Anuhea. He has a level 51 Gigalith. You get 1632 Pokédollars for winning.
Go south from the Hiker to find a PP Max.
Go back into the cave and go back the way you came until you see a Veteran to the east. Go east to battle Veteran Ella. She has a level 53 Pyroar, a level 53 Milotic, and a level 53 Claydol. You get 4664 Pokédollars for winning.
Go east from the Veteran, and the path will split. Go north at the split. The path will lead you to a Full Restore.
Go back to where the path split, and this time keep going south to find a Z-Crystal pedestal. Check on it to get an Icium Z.
Go west past the pedestal and jump down the one-way ledge, then go west through the narrow path. Go north from the Icy Rock to battle Ace Trainer Jada. She has a level 52 Vanilluxe and a level 53 Mismagius. You get 3604 Pokédollars for winning.
Go north and you will battle Master & Apprentice Breon and Kaimana. They have a level 52 Vikavolt, a level 53 Glalie, a level 53 Tyranitar, a level 53 Forretress, and a level 53 Bisharp. You get 8268 Pokédollars for winning.
Go north to leave the cave. There is another elevator up ahead, but before you go up, go west through the grass to find a Max Revive.
Then go up the elevator.
Mount Lanakila Summit
If you talked to the Aether Foundation person in Route 8 until she took the Vulpix to its home, you will find them here.
In the Mount Lanakila Pokémon Center, you can talk to the woman on the right, Madam Memorial, to teach your pokémon moves that they learned but do not know anymore, or moves that they could have learned on level up, but did not. You have to give her a Heart Scale for each move that she teaches to your pokémon. Heart Scales are often carried by wild Luvdisc, which are common when fishing in Route 9 on Akala Island. You can steal the Heart Scales from Luvdisc by using the move Thief, which you can teach using TM46 Thief. TM46 Thief is in the trial room of Verdant Cavern on Melemele Island. The pokémon that attacks with Thief will steal the target's held item as long as the attacker is not holding an item.
There is a Veteran west of the Pokémon Center who will battle you if you have defeated all of the Skilled Trainers around Alola. He can tell you where they are.
There is a hidden PP Max behind him.
The path to the Pokémon League is to the west. As you go north toward the Pokémon League, you can find a Poké Finder spot to the right.
Go north and there will be a cutscene. If you have your starter pokémon with you, there will be an additional cutscene.
The Pokémon League is to the north. When you go in, you won't be able to leave until you have either been defeated or you have become the Champion. You will not be able to use a PC Box, so make sure that the pokémon that you have with you are strong enough and are able to defeat a wide variety of pokémon. You will not be able to talk to a nurse to heal your pokémon between battles, so buy plenty of healing items, such as Potions and Revives. Try to get some Leppa Berries so you can replenish the PP of your pokémon's moves between battles. Leppa Berries can sometimes be found in berry piles on Melemele Island, and you can grow more in Poké Pelago. Give your pokémon Z-Crystals or other held items to help them during the battles ahead.
Alola Pokémon League
Go through the door to the left to battle Molayne. Elite Four Molayne has a level 56 Klefki, a level 56 Metagross, a level 57 Alolan Dugtrio, a level 56 Bisharp, and a level 56 Magnezone. You get 11400 Pokédollars for winning.
Step on the colorful platform behind Molayne to go back to the central room.
To the northwest, you can battle Kahuna Olivia. Elite Four Olivia has a level 56 Armaldo, a level 56 Probopass, a level 56 Gigalith, a level 56 Cradily, and a level 57 Midnight Form Lycanroc. You get 11400 Pokédollars for winning.
Step on the colorful platform behind Olivia to go back to the central room.
To the northeast, you can battle Captain Acerola. Elite Four Acerola has a level 56 Banette, a level 57 Palossand, a level 56 Dhelmise, a level 56 Froslass, and a level 56 Drifblim. You get 11400 Pokédollars for winning.
Step on the colorful platform behind Acerola to go back to the central room.
To the east, you can battle Elite Four Kahili. She has a level 56 Braviary, a level 56 Hawlucha, a level 56 Mandibuzz, a level 56 Baile Style Oricorio, and a level 57 Toucannon. You get 11400 Pokédollars for winning.
Step on the colorful platform behind Kahili to go back to the central room.
If you have defeated all four of the Elite Four, step on the colorful platform in the middle of the room to go to the next room.
Go up the stairs and check on the throne. You will battle Pokémon Trainer Hau. He has a level 59 Alolan Raichu, a level 58 Vaporeon, a level 58 Tauros, a level 59 Crabominable, a level 58 Noivern, and a level 60 starter that is weak to your starter (Primarina if you chose Rowlet, Decidueye if you chose Litten, Incineroar if you chose Popplio). You get 14400 Pokédollars for winning. Congratulations!
Press a button to receive a stamp in your Trainer Passport for becoming Champion, then press a button and watch the ending scenes. Be sure to watch all the ending credits so your game will be saved and you can start the post game the next time you play! Press a button when the screen says THE END and you will be returned to the title screen.