Having beaten Team Skull on their own turf, you should return to the Aether House on Route 15 to report back. Unfortunately, while you were absent, things have taken a turn for the worse.
Team Skull Gladion
Another Golbat; you know the drill. This one is a bit tougher though since it knows Acrobatics, which deals a lot of damage when the user has no held item. Rock, Electric or Steel-types are thus preferable.
Sneasel receives 4x damage from Fighting-types, so use those if you can. Otherwise, Bug, Rock, Steel, Fire and Fairy-types are good, althoug Rock and Fairies need to beware of Metal Claw.
Type: Null is to be expected; although now that most of your Pokemon have evolved, it should be easier to handle. Since it's a Normal-type, Fighting-types are recommended. Or you can try and hold it off with a Steel-type.
Afterwards, Gladion will offer to take you straight outside the ferry terminal in Malie City. If you prefer, you can refuse and head there yourself in your own time. In case you're unsure, it's where you first arrived in Ula'ula Island.
There, an unexpected visitor will come to send you off.
Island Kahuna Nanu
Note: Nanu has brought along some Hyper Potions and Full Heals.
Don't take it too serious, right... Definitely don't listen to his advice.
Sableye is only weak to Fairy-types. However Nanu's Sableye only knows Rock and Ghost moves, so Pokemon that resist those moves should do well. Just be prepared for the inevitable turn one Fake Out. Rather than waste your first turn, use an item if you need to.
Krokorok is weak to Grass, Water, Ice, Fairy and Fighting-types; that's a lot! It has the Intimidate Ablity, which lowers the opponent's Attack when it comes out. In terms of moves, it has the annoying Swagger, which raises Attack and causes confusion.
Dark Garfield... ahem, Alolan Persian is weak to Fighting, Fairy and Bug-types, but Fur Coat will reduce the damage it takes from physical moves. It also has Darkium Z, so Pokemon that resist Dark are ideal. It too has Fake Out and a Rock move in Power Gem.
Shine light on Nanu's darkness and you'll earn his Darkium Z, signalling the completion of your island trials on Ula'ula Island. From this point on, all Pokemon up to Level 65 will obey your command. Sadly, the celebrations will have to be put on hold though.