Upon reaching the Ruins of Life, head towards the centre, just before the entrance to the actual ruins. Lillie will catch up to you and kindly heal your party. Afterwards, Olivia will also arrive on the scene. If you feel you're ready, accept her challenge--the hardest one so far!
Island Kahuna Olivia
Olivia's Pokemon are all pure Rock-types, so Grass, Water, Fighting, Steel and Ground Pokemon should be among your party.
Nosepass has Spark and Thunder Wave, which are bad news for Water-types. Send a Ground-type after it if at all possible.
Boldore has Mud Slap, which is strong against Steel-types. The damage itself isn't too noticeable, but the accuracy drop can be painful. It also has Sturdy, so no one-hit KOs.
Olivia's final Pokemon, Lycanroc is plenty menacing, but not too fearsome if well-prepared. It knows Bite, so be careful of the annoying flinches. It also has Rockium Z, so Pokemon that resist Rock moves (eg. Fighting, Steel and Ground-types) should be fielded.
Overcome the mountain and you'll earn Rockium Z . Congratulations, you've passed your second grand trial! Pokemon up to Level 50 will now obey you without question, so you can finally use the Ash-Greninja from the Special Demo if you desire a powerful Water-type.
Afterwards, you'll be taken outside. Having completed the grand trial, you can now head to Hano Grand Resort to see what that weirdo from the Aether Foundation was babbling about.