Chapter 15: Pokemon League
Head north out of Tapu Village towards the Pokemon League! A cutscene will play and you'll be greeted by Gladion who will once again want to battle. He has a Lv. 52 Crobat, a Lv. 53 Silvally, a Lv. 52 Weavile, and a Lv. 52 Lucario. He's pretty challenging, so you may need to do some training before you are able to beat him. He'll give you a Max Elixir once he is defeated. Heal up if you need, and then head up the elevator.
Pick up the Escape Rope in the Item Ball to your right and the Zygarde Cell a little further ahead to the left. Continue into the next area and forward until you come to a fork. Go left at the fork for TM13 - Ice Beam. Head back at the fork and continue forward. Head down the path to the right and you'll find a Z Pedestal. Grab the Icium Z from it. You'll also find a Full Restore nearby. Continue on the left-most path and exit the cave.
Grab the Max Revive to your left and then head up the elevator. Heal up at the Pokemon Center here and continue forward to the left. Grab the Zygarde Cell by the rock and continue north. Hau will stop you after a while and challenge you to a Battle. He's got a Lv. 53 Raichu, a Lv. 53 Flareon, a Lv. 53 Komala, and a Lv. 54 starter. He probably won't give you as much trouble as Gladion, but he'll definitely put up a fight. He'll give you 3 Max Revives after you've won. Heal up, then continue forward and you'll run into Professor Kukui. He'll give some dialogue about the Pokemon league. Once you're ready, enter through the doors.
Elite Four Hala
Enter the first door on the left. You'll face off against Hala, who specializes in Fighting types. He has a Lv. 54 Hariyama, a Lv. 54 Primeape, a Lv. 54 Bewear, a Lv. 54 Poliwrath, and a Lv. 55 Crabominable. A strong Flying type with a move like Hurricane should make quick work of his team. Psychic types are also strong options, but tend to be slightly weaker defensively, so watch out for his hard hitting moves like Outrage.
Elite Four Olivia
Head into the next door to fight Olivia, who specializes in Rock type Pokemon. She has a Lv. 54 Relicanth, a Lv. 54 Golem, a Lv. 54 Carbink, a Lv. 54 Probopass, and a Lv. 55 Lycanroc. A Grass type Pokemon will take out everything but Probopass, which you'll probably want a Fighting type for. Watch out for Reflect and Sandstorm that will make the battle more challenging.
Elite Four Acerola
Head into the next door to battle against Acerola, the Ghost type specialist. She has a Lv. 54 Sableye, a Lv. 54 Froslass, a Lv. 54 Palossand, a Lv.54 Drifblim, and a Lv. 55 Dhelmise. A Dark type is recommended alongside Pokemon that know strong Fire, Water, and Electric moves to take care of secondary typings. She's very liberal with Full Restores, so watch out for that.
Elite Four Kahili
The final member of the elite four is Kahili, who specializes in Flying type Pokemon. She has a Lv. 54 Skarmory, a Lv. 54 Mandibuzz, a Lv. 54 Toucannon, a Lv. 54 Crobat, and a Lv 54 Oricorio (Baile Style). A strong Electric, Rock, or Bug type will work really well. Watch out for the Spikes that Skarmory will lay that deal damage every time you switch a new Pokemon in.
Pokemon League Champion
Prepare your team with revives and potions and then walk into the center teleporter. In order to become the champion you must fight Professor Kukui. He has a Lv. 57 Lycanroc, a Lv. 56 Braviary, a Lv. 56 Snorlax, a Lv. 58 Incineroar, a Lv. 57 Magnezone, and a Lv. 56 Ninetales. His team is varied, but a good Fire, Figthing, and Water type will take out most of his Pokemon. He's got a lot of Full Restores, so don't count those out.
Congratulations on becoming Alola's first ever champion and beating Pokémon Sun or Moon version! Once you've beten Kukui, watch the long series of cutscenes until Lillie eventually takes you to meet Tapu Koko and battle against it. If you accidently kill it now, you'll be able to get another shot at catching it in the post game, so don't worry too much. Either way, you'll get Tapunium Z. During the ending cutscenes you will also receive a Poké Doll form Lillie. Continue watching the cutscenes and credits and makes sure to join us for post-game, your journey isn't over yet!