After you go through Route 8 and explore the northern part of Route 5, you continue on to Lush Jungle for your third Akala Island trial.
From the entrance of Lush Jungle, go north to talk to Captain Mallow. If you're ready, say "I'm ready!" and the trial will start.
Mallow wants you to gather four ingredients for her: Mago Berry, Honey, and a Big Root. She gives you a Forage Bag to put the collected items into.
First, she takes you to an area where you can find Mago Berries, but she warns that the Fomantis in the area might battle you if you try to take the berries. She says that Fomantis live in trees that are dripping with Honey, so if you don't want to be attacked, look for a tree that has a sour scent. Check on each sparkle to find out what scent the tree has.
Next, Mallow takes you to an area where you can gather Honey, but she says that pokémon gather near the flowers, so you might be attacked there if you try to approach.
Go to the north part of the area to find a Max Revive.
Go to the west side of the area and check on the tree to crawl through to the previous area. You can get TM86 Grass Knot here.
Crawl back through the tree, then go southeast. There is a hidden Revival Herb in the short patch of grass there.
Go to the east side of this area and use Stoutland to help find a hidden Hyper Potion.
Then check on the flower bushes. Look for the flower bush that only has a few flowers. Gather the honey from that bush to avoid being attacked.
Then Mallow takes you to an area where you can find a Big Root. You can check on the Sudowoodo-like trees to find one, but some of them really are Sudowoodo instead of trees. Look carefully at the Sudowoodo's hands and you will find that one of them has leaves for hands instead of round green spheres. Check on the Sudowoodo with leaf hands to get the Big Root without being attacked.
Go east to find an X Defense.
Go to the east side of the area, then go down the south path, and you will find Captain Lana. Near her, use Stoutland to help find a hidden Heal Powder on a dark patch of soil.
After you get the Big Root (see the explanation in a previous paragraph for tips) talk to Mallow to give it to her.
After Mallow makes the meal, the Totem Lurantis attacks. Lurantis is weak to Flying, Poison, Bug, Fire, and Ice attacks. It can call other pokémon into battle.
After you defeat the Totem Lurantis, Mallow gives you the Grassium Z-Crystal. She also gives you 10 Nest Balls.
After that, Professor Kukui gives you TM67 Smart Strike. Then he asks you to go to the Dimensional Research Lab. He also mentions that Olivia uses Rock type pokémon, and recommends that you use Grass types against them. Other types that work well against Rock-type pokémon are Water, Fighting, Ground, and Steel. Avoid bringing Fire, Ice, Flying, and Bug pokémon into battle against Rock types, because they are weak to Rock attacks.
Dimensional Research Lab
The Dimensional Research Lab is in Heahea City. You will find Lillie there. After a cutscene, go into the building and check on the elevator buttons to go to 3F.
In a cutscene, you learn about Ultra Wormholes from Professor Burnet. After the cutscene, talk to Lillie.
You can check on the books in the bookshelves here to learn more about certain pokémon.
When you are ready, go up to Professor Kukui in front of the elevator. After the cutscene, check on the elevator buttons to go to 1F. Then leave the building. After the cutscene, you can go to Diglett's Tunnel when you're ready. The entrance is across from the Surf Association building.