Walkthrough: After completing Captain Mina's Trial, you can head back to Exeggutor Island to have your Poni Island Grand Trial with Kahuna Hapu. If you didn't already know this, Kahuna Hapu is a Ground Type Pokemon User and has access to Z Crystals. Defeat her to receive your next stamp of your passport - allowing Pokemon up to Level 80 to obey you - as well as Groundium Z.
Kahuna Hapu - Level 53 Golurk, Level 53 Gastrodon, Level 53 Flygon, Level 54 Mudsdale
Once you have completed your fourth and final Grand Trial, it is time to head to Mount Lanakila to challenge the first ever Alola Region Pokemon League.