After completing Acerola's Ghost-type trial, you go back to Aether House in Route 15.
Revisit Aether House
Next, you should go back to Aether House. When you get there, you have to battle Team Skull Admin Plumeria. She has a level 34 Golbat and a level 35 Salazzle. You get 4200 Pokédollars for winning.
Afterward, Plumeria says you have to go to Po Town to face Team Skull by yourself. To get there, Acerola says to look for a guy on Route 15 who is wearing a kimono.
You get a Rare Candy.
Explore Route 15
The girl at the start of the route will battle you if you defeat all the trainers in the route. She is Trial Guide Katrina, and she has a level 34 Klefki and a level 34 Skarmory. You get 1632 Pokédollars for winning. She also gives you a PP Max.
Outside, go west and go up through the grass to find an item.
South of this grass, you have to battle Ace Trainer Yuki. He has a level 32 Sandshrew and a level 33 Marowak. You get 2244 Pokédollars for winning.
Go down the ramp near the Ace Trainer. There is a green sparkle for the Zygarde Cube south of the ramp.
The man in the kimono is here on the beach, so talk to him. He will ask you what the result of his coin toss will be. Say Neither. He will add Sharpedo to your Ride Pager. This will allow you to break through boulders in the water. He says that Po Town is at the end of Route 16, which is north of Route 15.
With Sharpedo, you can go back to Melemele Sea on Melemele Island and break through the boulders in the water there. This gives you access to the south side of the island, where you will find TM80 Rock Slide.
From the beach where you spoke with the man in the kimono, ride Sharpedo southwest and break through the boulders there, then go south, west, then north.
There is a narrow island to the right. On this island, you can battle Ace Trainer Karla, who has a level 34 Vulpix, a level 34 Haunter, and a level 35 Wigglytuff. You get 2380 Pokédollars for winning.
North of the Ace Trainer, you can get TM93 Wild Charge.
Go south to get back into the water. Go north and you can battle Swimmer Jared. He has a level 33 Tentacruel. You get 792 Pokédollars for winning.
Go northeast and you will find a beach. There, you can battle Swimmers Yumi and Jake. They have a level 33 Slowbro and a level 33 Golduck. You get 1584 Pokédollars for winning.
North of the swimmers, there is a Dive Ball.
Get back in the water and go west. You can battle Swimmer Alexandria. She has a level 33 Luvdisc, a level 33 Alomomola, and a level 33 Corsola. You get 792 Pokédollars for winning.
Go onto the large beach above you. There is a hidden Normal Gem on the far left side of the beach.
There is a hidden Pearl near the far right side of the beach.
Then go north up the slope to reach the Route 16 Pokémon Center.