Chapter 11: Aether Paradise
As soon as you've beaten Kahuna Nanu, a cutscene will take you back to Aether Paradise. Head left and north to battle a trainer. Keep heading north past three more trainers. You can avoid them if you want, but we recommend defeating them for the expirience. Before getting on the elevator, head north for a Rare Candy. South and to the left you'll find an X Sp. Atk and TM06 - Toxic. There's another trainer to the left of the elevator that you can battle if you'd like, otherwise hop on the elevator.
Gladion will take you to an upper floor and you'll immediately be greeted by the esteemed Branch Manager Faba who will want to battle. He has a Lv. 39 Hypno. You shouldn't have much trouble beating it, but it can put you to sleep and disable your moves, so watch out for that. Faba will then agree to take you to the lower levels, but say you aren't ready at first. You can head south a little and speak to the lady behind the counter to have your Pokemon healed. Now tell Gladion that you're ready.
When you get to the lower floor, Gladion will tell you to go to Secret Lab B and Hau will give you a Max Revive. Head right through the door. You'll be greeted by three Aether Foundation employees that you'll have to defeat in battle. After defeating them continue all the way to the right and pickup the Zygarde Cell before entering Secret Lab B. Once inside, you and Hau will immediately face off in a double battle against a couple of employees. After you've taken them out, investigate the computer. Exit the room and then enter Secret Lab A. You can find a Full Restore here and read some interesting lore about Type: Null and read a blog by Faba. Head back left to the elevator. Head up to 1F.
You'll be confronted by some Aether Foundation employees and have to take them on in another double battle with Hau. After you defeat them, more dialogue will play and Wicke will appear and heal your Pokemon. Head north after Gladion and Hau. After Hau roasts Faba a bit, you'll be pit against two more Foundation employees in a double battle with Gladion. Immediately after (you're Pokemon will be healed first) you'll face off against Faba and another employee in another double battle, this time with Hau. Faba will have a Lv. 39 Slowbro, a Lv. 39 Bruxish, and a Lv. 40 Hypno. Exit through the entrance.
Head left and south to pick up a Full Heal. Head back right and to the south and you'll find a Zygarde Cell. Head back through the middle and start facing Team Skull grunts. You'll have to beat two before getting to Gladion and Guzma. You'll then face off against Guzma. He has a Lv. 41 Golisopod, a Lv. 40 Masquerain, a Lv. 40 Ariados, and a Lv. 40 Pinsir. After defeating him, head left around the building to find a Zygarde Cell. Enter the doors when you're ready.
There's another Zygarde Cell under a shelf on the right side of this room. Enter through the center door. A cutscene will play where you learn that Lusamine is the mother of Lillie. Follow Lusamine through the teleporter. Here Hau and Gladion will join you and you'll find that Gladion is the son of Lusamine. A long cutscene will continue and Ultra Beasts will be released into Alola. You'll have to face off against Lusamine. She has a Lv. 41 Clefable, a Lv. 41 Milotic, a Lv. 41 Mismagius, a Lv. 41 Lilligant, and a Lv. 41 Bewear. After you've beaten her, she'll disappear into the worm hole with Guzmo. Another cutscene will play and you'll be all healed up.