You'll find the entrance to the canyon if you head north from the central crossroad in Ancient Poni Path. Attempt to head north though and you'll be stopped by a disrespectful bunch. Haven't these guys learned their lesson already?
Team Skull Grunt
After receiving yet another sound thrashing, the grunts become desperate and band together for one final stand.
Team Skull Grunt
Salandit against a Ground-type is a total wipe out, while Water, Rock and Psychic-types are typically effective. It's unevolved, but don't take it lightly and definitely don't let it set up using Toxic or Nasty Plot.
Mareanie is another unevolved Pokemon, but bad news if you're weak to it. For best results, send a Psychic or Electric-type after it. Or a Ground-type since it doesn't have any Water-type moves.
Haunter is an evolved Pokemon, albeit not a fully evolved one. It's weak to Dark, Ghost and Psychic-types, but the latter too won't like Haunter's Ghost and Dark-type moves. Ground-types are ineffective, unless they can bypass Levitate; Zygarde can do this with Thousand Arrows.
Golbat's in the same boat as Haunter. You shouldn't need the details, but it's weak to Electric, Rock, Ice and Psychic-types. If your Pokemon were poisoned earlier, it can use Venoshock to double the pain.
Finally, Raticate is simply no match against Fighting-types and easily dispatched with Fairy and Bug-types. It has Super Fang, which halves your current HP and Endeavour, which reduces your Pokemon's HP to Raticate's HP.
Despite everything, the Skulls are obviously no match to you. Before things turn sour, their big sister comes along to calm them down. To apologise, she'll give you her prized Poisonium Z . After that little drama, Lillie will heal your team before rushing off ahead.
Vast Poni Canyon
From the entrance, head all the way north towards the big tree where Hapu and Lillie are waiting. Although you've yet to complete all of the island trials (your 7th trial is up in the air), Hapu challenges you to a battle as the newly-elected kahuna of Poni Island.
Island Kahuna Hapu
Flygon is the final evolution of Trapinch. Its Type combination makes it incredibly vulnerable to Ice-type moves--pretty much any decent move can one-shot it, so use those if you've got them. Otherwise, it's typically weak to Fairy and Dragon-types.
Gastrodon is extremely weak to Grass-types, immune to Electric-types and neutral or resistant to everything else. It's not the most threatening creature around, but with its decent bulk and Recover, it has the potential to outlast lesser opponents.
Finally, Hapu's trusty Mudsdale is extra formidable since it's fitted with Groundium Z, allowing to pull of a powerful Ground-type Z-Move. It has Counter and the Stamina Ability, so special attackers--especially those that resist or are immune to Ground--are highly recommended.
Stand up to Hapu's might and you'll receive her Groundium Z , which also means you've passed the grand trial of Poni Island. From here on, Pokemon up to Level 80 will obey you without question. However, since you're short one island trial, you haven't officially cleared the island challenge.
Well, you can figure out the specifics later; right now, you've got a mission ahead of you. Once you're ready to proceed, head north to where Lillie is. She'll give you a Max Revive and kindly restore your team. Before entering the cave ahead, check the tree branches to Lillie's west.
You'll find you can crawl underneath, where a Max Potion waits on the other side. Return back to Lillie and once you believe you're fully prepared, enter the cave to begin your long trek through the Vast Poni Canyon. You'll going very deep, so be sure you've brought adequate supplies!
After passing Hapu's grand trial, not a lot changes.
If you visit the floating restaurant towards the west side of Seafolk Village, you can trigger another kahuna meal event. Select the "Zumongous Z-Noodles" and Hapu will come to join you. You'll receive double the Heart Scales (so 2) without paying a dime.
You can repeat this event once a day, which is only useful if you're a fan of Hapu's or really need Heart Scales. Since you can get a lot more Heart Scales from dining with Nanu at Sushi High Roller (8) or Olivia at Mallow's restaurant (4).