After catching the legendary pokémon, you go to Mount Lanakila to challenge the Pokémon League.
Battle Gladion
When you go to Tapu Village and from there to Mount Lanakila, go up to the platform and you will have to battle Pokémon Trainer Gladion. He has a level 52 Crobat, a level 53 Silvally (Fire type if you chose Rowlet, Water type if you chose Litten, and Grass type if you chose Popplio), a level 52 Weavile, and a level 52 Lucario. You get 8480 Pokédollars for winning.
Afterward, you get a Max Elixir.
Go Up
Get on the elevator and press the button in the middle to go up.
To the right of the grass, there is an Escape Rope.
North of there, you find a green sparkle on the left.
Go north and enter the cave.
Go north up the path in the cave. When the path splits, go left to get TM13 Ice Beam.
Then go east, and at the split, go north.
Up ahead, the path splits again. If you go right, then south, you find a pedestal containing Icium Z.
After getting the Z-Crystal, go northeast and keep going north. The path curves to the left, and you find a Full Restore.
Keep going to the left and you find an Icy Rock, which means that if you level up an Eevee in here, it will evolve into Glaceon.
Go north from the Icy Rock to find an exit to the outside.
To the left of the grass, there is a Max Revive.
Go north and check on the button in the elevator to go up.
Go into the Pokémon Center. Now you will be able to fly back to this Pokémon Center any time.
In the Mount Lanakila Pokémon Center, you can talk to the woman on the right to teach your pokémon moves that they have forgotten. You have to give her a Heart Scale for each move your pokémon relearn. 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.
Outside, on the ledge to the left of the Pokémon Center, there is a hidden PP Max.
To the left of that, there is a green sparkle.
Go north and Hau will challenge you. Pokémon Trainer Hau has a level 53 Alolan Raichu, a level 53 Komala, a level 54 starter (Decidueye if you chose Litten, Incineroar if you chose Popplio, or Primarina if you chose Rowlet), and a level 55 Eeveelution (Vaporeon if you chose Litten, Leafeon if you chose Popplio, or Flareon if you chose Rowlet). You get 8640 Pokédollars for winning.
Afterward, Hau gives you three Max Revives.
Go north. There is a Poké Finder spot to the right.
Go north and you will meet Professor Kukui. After the cutscene, if you are ready, you can go north and enter the Alola Pokémon League.
Alola Pokémon League
To the left, you can challenge Kahuna Hala. Elite Four Hala has a level 54 Hariyama, a level 54 Primeape, a level 54 Bewear, a level 55 Crabominable, and a level 54 Poliwrath. You get 11000 Pokédollars for winning.
Step on the colorful platform behind Hala to go back to the central room.
To the northwest, you can battle Kahuna Olivia. Elite Four Olivia has a level 54 Relicanth, a level 54 Carbink, a level 54 Probopass, a level 55 Lycanroc, and a level 54 Golem. You get 11000 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 54 Sableye, a level 54 Dhelmise, a level 55 Palossand, a level 54 Frosslass, and a level 54 Drifblim. You get 11000 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 54 Skarmory, a level 54 Mandibuzz, a level 55 Toucannon, a level 54 Pa'u Style Oricorio, and a level 54 Crobat. You get 11000 Pokédollars for winning.
Step on the colorful platform behind Kahili to go back to the central room. Then 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 Professor Kukui. He has a level 57 Lycanroc, a level 56 Alolan Ninetales, a level 56 Snorlax, a level 56 Magnezone, a level 58 evolved starter that is strong against the starter that you chose (Primarina if you chose Litten, Incineroar if you chose Rowlet, or Decidueye if you chose Popplio), and a level 56 Braviary. You get 13920 Pokédollars for winning. Congratulations!
After Becoming Champion
After you become champion and watch the ending scenes, Lillie takes you to the Ruins of Conflict, where you must battle Tapu Koko. You should try to catch it! But if you don't, you can get another chance to catch it by defeating the Elite Four again.
Try throwing a Quick Ball on the first turn. You can't inflict sleep while Electric Terrain is in effect, so just bring the Tapu's HP down until the terrain effect disappears. Then use Dusk Balls if you have them, or Ultra Balls if you don't. If the battle has been going on for a long time, start throwing Timer Balls. It is not recommended to use your Master Ball.
When you catch Tapu Koko, you get Tapunium Z.
After this, watch the ending scenes. When the "The End" screen appears, press a button and your game will be saved.
The next time you load your saved game, the post-game will begin!