Final Gladion Battle

Walkthrough: After defeating Necrozma and returning the light to The Alola Region, as well as completing Captain Mina's Trial and Kahuna Hapu's Grand Trial, you will move forward with challenging the newly created Pokemon League. The Pokemon League can be found on the peak of Mount Lanakila on Ula'Ula Island. However, as you reach the base of the mountain, you will be met by Gladion. He will thank you for all of your help earlier in the story, and then he'll challenge you to showcase his strength and to see why Lillie believes your player is the strongest in the world. This will be your final battle with Gladion in the Main Story, and his newly evolved Silvally will be holding the memory that your Starter is weak against.

Pokemon Trainer Gladion - Level 53 Crobat, Level 53 Zoroark, Level 53 Lucario, Level 55 Silvally

After defeating Gladion, he will mention that even though you are neither enemies or friends, he appreciates everything you have ever done. Before leaving, he will mention that Lillie is back with Solgaleo in Mahalo Trail on Melemele Island.