Walkthrough: The fourth Island Trial, the final one on Akala Island, is found in Lush Jungle and is set by Captain Mallow. In this Trial, you will be given the Forage Bag by Mallow and you must search for a Mago Berry, a Tiny Mushroom, a Revival Herb and a Miracle Seed that are hidden in the Jungle, meaning you must use Stoutland to help you.
As you go through this small hunt, you will be attacked by a Level 22 Parasect, a Level 22 Fomantis and a Level 23 Formantis after finding the Tiny Mushroom, Mago Berry and Revival Herb, respectively. Find and collect the final ingredient of the Miracle Seed, and Kiawe and Lana will join both you and Mallow.
After Mallow cooks her meal with the ingredients you fond and the other Captain brought with them, the Totem Pokemon will come to Battle you, a Lurantis at Level 24. It's Speed will be boosted by one stage as the Trial begins, with the ability to call for help from both Castform and Trumbeak (all at Level 22). Some of the Pokemon Lurantis calls will even use the move Sunny Day, furthering its own strengths.
After you defeat the Totem Pokemon and all the Pokemon it calls upon for assitance, you will complete the final Island Trial of Akala Island. You will obtain Grassium Z from Mallow, and then will be prompted to go meet up with Lillie and Kukui at the Dimensional Research Lab just east of Heahea City.