As you arrive back in Iki Town, the others will be waiting for you. Kukui gives you a useful tip, which is Hala is an expert of Fighting-type Pokemon. That means Flying, Fairy or Psychic-types will be your best friends.
Anyway, head on over and chat to Hala by the base of the stairs. He'll ask if you're ready to battle. If your Pokemon got hurt along the way, they will be magically healed before the battle. However it might be wise to check your Pokemon's held items etc. before accepting the challenge.
Island Kahuna Hala
Note: Hala has access to a Super Potion.
This is probably your toughest battle so far, but not unwinnable by a far margin. All of old Hala's Pokemon are Fighting-types and nothing except, so it'll be a smooth victory if you pick the right Pokemon types.
As we hinted earlier, Psychic, Flying or Fairy Pokemon are your best bests as their moves are super-effective, while Fighting moves are not very effective against them. Ghost Pokemon are also good because they're typically immune to Fighting moves.
If you chose Rowlet at the start, this battle is mostly an exercise in pecking your opponents to death. Likewise, if you caught Oricorio and trained it a little, it'll have a similarly easy time. Psychic types should be wary of Crabrawler's Pursuit, but that's about it.
Succeed and you'll receive the Fightium Z, which is obviously the Fighting-type Z-Crystal. With this in your possession, you've officially passed the grand trial of Melemele Island! In addition, all Pokemon up to Level 35 will obey you even if they're from a trade.
Afterwards, you'll get a Ride Pager from Hala and Tauros Charge added as a Ride Pokemon option. Whenever you're out on the field, you can press the Y button to use the Ride Pager and instantly summon a Ride Pokemon. You can also set D-Pad shortcuts for each Ride Pokemon.
Kukui isn't a cheapskate either and he gives you TM54 False Swipe as a gift. If you ever worried about accidentally fainting a Pokemon, False Swipe will always leave a Pokemon with at least 1 HP. It doesn't work on Ghosts though, unless you use Foresight, etc.
When it's time to move on, head for the Marina in Hau'oli City. With Tauros Charge, you should be able to get there pretty quick. If you don't remember the way, refer to the bottom screen map. At the Marina, Kukui and Lillie will be waiting to take you to the next island.
Around this time, a bunch of sidequests and side areas will open up on Melemele Island. They will be covered in the next section, but we recommend continuing onto Akala Island regardless. Instead, come back after you've acquired the Zygarde Cube.
You can wander around Melemele Island now if you want. But after you get the Zygarde Cube, you'll probably want to head back to Melemele Island anyway. So rather than tour the island twice, it makes more sense to do it all in one fell swoop!