Walkthrough: After defeating Team Skull one last Time, you will gain access to the path just north of Ancient Poni Path. Go through the path, and you and Lillie will quickly find yourself just outside the Vast Poni Canyon. Standing by the tree outside the Canyon will be Hapu, who was now given the title of Poni Island Kahuna from Tapu Fini. And with no better time than the present, she will challenge you to your final Grand Trial, even before you take on the one and only Island Trial. Kahuna Hapu is a Ground Type Master.
Kahuna Hapu - Level 47 Dugtrio, Level 47 Gastrodon, Level 47 Flygon, Level 48 Mudsdale w/Groundium Z
After defeating her, you will be given Groundium Z. You will also be given the next stamp on your passport, which will make all Pokemon up to Level 80 to obey. Defeating her, as well as the final Island Trial in Vast Poni Canyon, will officially complete your Island Challenge, which ultimately will later give you the right to challenge the newly created Pokemon League.