Chapter 10: Po Town
Head back north through Tapu Village and heal up at the Pokemon Center here. Head left to Route 15 and back north to the Aether House. You'll find Hau, Plumeria, and some Team Skull grunts here. Plumeria will challenge you to a battle. She has a Lv. 34 Golbat and a Lv. 35 Salazzle. She's pretty tough, but you should be able to take her out if you can get the type advantage. Some dialogue will play out and you'll get a Rare Candy. Heal up if you need to, then head left on Route 15. Grab the Hyper Potion and defeat the trainer here. There's a Zygarde Cell on the south side of the beach you can pickup. When you're ready, speak to the man in white.
He'll reveal himself as Grimsley. He'll add Sharpedo to your Ride Pager. Use Sharpedo to start riding accross the sea to Route 16. Smash the rocks to the south and continue left. Hop off onto the land in the midde island and defeat the trainer. Grab TM93 - Wild Charge in the Item Ball to the north. Get back in the water and head north. Get out on the beach on the right side of the cliff and grab the Dive Ball. You can challenge the couple doing crunches here to a double battle if you want as well. Now go to the left beach and ascend the cliff to reach the Pokemon Center and heal up here. There is a Zygarde Cell to the left of the Pokemon Center doors, but note this may only appear during the day cycle.
Head outside and grab the berries under a tree to the left. Head right and crawl under the building to find TM81 - X-Scissor. Enter the building to the right of the Pokemon Center (you just crawled under it) to find Sina and Dexio who gave you your Zygarde Cube. They'll explain the process of gathering and assembling Zygarde Cells and Cores into different formes of Zygarde. Exit north to Ula'ula Meadow.
There will be a Zygarde Cell on the right side of the dock at the beginning of the route. A little further up, get off the dock and face the trainer, you'll find some Red Nectar and a Repeat Ball next to her. There are trainers all over the dock that you can battle if you'd like. On the north and left side you can pick up a X Sp. Def, an Adrenaline Orb, and some Red Nectar. On the far right and north side of the route you'll find a Zygarde Cell. Head north and exit the meadow.
Take the right side of the route, up a cliff, if you want some goodies. On the upper level you'll find a Lemonade, a Full Heal, and a Team Skull grunt. Hop down to the right for a Zygarde Cell. Defeat the grunt here and hop back down. Head south again, defeating the grunt on your way and wrapt back up to the top of the cliff. This time, hug the left side to get TM84 - Poison Jab. Head north up the route, grabbing the Zygarde Cell on the left and defeating the grunt on the right. Continue on the path to the entrance to the town.
You'll be challenged to a double battle by two grunts. After you've beaten them, a mysterious man will let you into Po town. Head to the right and pickup the Max Repel from an item ball. Then head left and crawl through a whole in the fence to get past the barricade. You'll then have to face both grunts in battle. You can heal up at the Pokemon Center, but Team Skull grunts have taken it over so you'll have to pay 10 Poké Dollars. Enter through the right hole this time and you'll be put in a double battle against two Team Skull grunts. In this area you'll find a Rare Candy.
Now, crawl through the left hole and defeat the grunt on top of the truck. Enter through the left and go all the way to the right and enter the hole in the hedge back towrads the entrance of the town. In this little cranny you'll find a Smoke Ball. Head back left and defeat the patrolling grunt. You can grab a Zygarde Cell from the area up North, then head towards the doors at the end of town.
Inside the Shady House, take the door on the far left to go back outside and pickup an Ice Stone. Re-enter the building and take out the grunt in the left hallway. Enter the south door for a Max Revive and you can find out the Team Skull password in the north door "Beat Up". Enter the room left of the staircase to face another grunt. Exit through the door on the right and enter the next room to find a second password, "Tapu Cocoa" and a Heal Powder. Back track and challenge the remaining grunt on the first floor if you'd like. Afterwards, head up to the second floor.
Go left and enter the north door to find TM36 - Sludge Bomb. Enter the middle door if you want to battle two grunts. Enter the south door to face a grunt and get an Iron and a 3rd password: "Golisopod". Defeat the patrolling grunt and then speak to the grunt blocking the door to the south. Select the three correct passwords ("Beat Up", "Golisopod", and "Tapu Cocoa"), but when you're asked if you're sure, say "no". Go through the door and pick up the Escape Rope outside. You can also battle the grunt here if you want. Either way, enter in through the window. Go straight to the next door.
Inside, you'll finally meet up with Guzma. You'll battle him, of course. He has a Lv. 37 Goliosopod and a Lv. 36 Ariados. You shouldn't have much trouble beating him if you're Pokemon are in the 38-40 level range and you've been battling most trainers up to this point. After you've defeated him, investigate the chest to get Buginium Z.
Exit the building and a cutscene will play with the mysterious man who is identified as Nanu. Use Charizard to fly back to Tapu Village. Now had left back to the Aether house. A cutscene will play when you enter and it is revealed that Lillie was taken by Plumeria. Gladion will then barge in and battle you. He has a Lv. 37 Golbat, a Lv. 38 Type: Null, and a Lv. 37 Sneasel. After you defeat him, fly to Malie City and heal up. From here, head right to the Ferry Terminal and another cutcene will play.
Island Kahuna Nanu
Kahuna Nanu will show up and offer to battle you. He's got a Lv. 37 Sableye, a Lv. 38 Krokorok, and a Lv. 39 Persian. His Persian is quite tough, but by no means unbeatable. When you beat him, he'll give you Darkium Z and your Grand Trial will be complete.