Walkthrough: Your final Island Trial on Ula'ula Island takes place within the Abandoned Thrifty MegaMart at the end of Route 14, and it's set by none other than Captain Acerola. This trial involves you searching through the building and taking pictures via your Poke Finder of three Ghost Pokemon. These Ghost Pokemon include Gastly, Haunter and Gengar, all of which are at Level 30.
As you take these pictures of all three Pokemon, a ghost Pikachu will slowly lead you to the backroom. At it, you will see an image of Acerola who insists you leave this place. After refusing to leave, this image of Acerola will disappear and the backroom will open. Once in the backroom, which you later learn from the real Acerola doesn't exist, you must take a picture of the Totem Pokemon in order to battle it. The Totem Pokemon is a Level 35 Mimikyu, and as the battle commences, all of it's stats will increase by one stage. Mimikyu will also be able to call upon both Banette (Level 32) and Jellicent (Level 33) for help.
After defeating the Totem Pokemon and all the Pokemon it calls upon for help, your final Ula'ula Island Trial will be complete. You will obtain Ghostium Z from Acerola, and the two of your, plus Hau, will return to the Aether House.