Akala Island Grand Trial

Walkthrough: After defeating Plumeria and getting to the end of Akala Outskirts, you will arrive at the Ruins of Life. Directly outside of them is Kahuna Olivia, the woman you met right after arriving on Akala Island. Immediately after you arrive there, Kahuna Olivia will challenge you to your Grand Trial. Lillie and Burnet will arrive before the Battle begins, with Lillie actually watching as you face Olivia. She is a Master of Rock Type Pokemon.

Kahuna Olivia - Level 26 Nosepass, Level 26 Boldore, Level 27 Midnight Lycanroc w/Rockium Z

After defeating her, you will earn the next Stamp on your Passport that will cause all Pokemon up to Level 50 to obey you; she will even give you the Rockium Z-Crystal. After the Battle ends, Hau will challenge Olivia to his Grand Trial. As he battles her and Lillie continues to watch for his sake, you will go to the Grand Hotel Resort to Talk to that Aether Employee.