Coming from the impact crater, you'll arrive in another icy cavern. This is where the path along the south-eastern side of Mount Lanakila would have led to, if the rocks weren't blocking the way. Directly to the east is an icy rock. If you Level Up Eevee around here, it'll evolve into the Ice-type Glaceon. There's also a trainer circling this rock.
Emolga is a flying squirrel with high Speed, but otherwise middling stats. For attacks, it can use Discharge and Volt Switch to deal STAB damage, Encore to cause your Pokémon to keep repeating its last move, and Agility to boost its Speed. Rock and Ice-types are recommended.
North from here is the exit, being guarded by a female Ace Trainer. If you're in a rush, head that way. Otherwise, go south from the icy rock if you wish to fully explore the mountain. Along the way, you'll need to squeeze through a narrow path that goes south-east. When the path widens, there's a trainer above.
Pyroar is an evolved Litleo with high Special Attack and Speed. Ella's Pyroar focuses on offense, using Flamethrower and Hyper Voice to deal STAB damage. Ideal Types to use are Water, Rock, Ground and Fighting.
Claydol is an evolved Baltoy with bulky defenses and the Levitate Ability. This one is also offensively built, able to use Earth Power and Extrasensory to deal STAB damage. Grass, Water, Ice, Ghost and Dark-types are recommended.
Finally, Milotic is an evolved Feebas with exceptional Special Defense and good Special Attack. Unsurprisingly, this Milotic only carries attacking moves: Hydro Pump to deal heavy STAB damage, and Blizzard to punish Grass-types. Electric-types are ideal.
Run past Ella, towards the north-east. When the path branches, follow the path to the south-east. You'll come to a pedestal containing Icium Z , which is of course the Ice-type Z-Crystal. This should be the final Type Z-Crystal you'll obtain (and the last one that appears in the save menu), but not the last Z-Crystal you'll see.
After warming up your hands, you can either immediately go backwards to the fork in the path, or continue west from the pedestal. If going west, leap off the ledge at the end, which takes you right next to Ella, then run back to the fork from there. Either way, follow the path to the north this time, and grab the Full Restore at the end.
Having explored this side of the cavern, return to Ella and follow the path leading south. After coming down the slope, the path will branch off towards the west and the east. Head west first, along the narrow path. At the end of this short path, you'll find TM13 Ice Beam on top of a ledge. After this cool find (sorry), travel east.
The path towards the east will start to bend slightly south. Along the north, there are two small rocks below the cavern wall. When you're positioned approximately halfway between these rocks, stop. Towards the east is a Pokémon trainer looking west and south. If you wish to avoid him, run past when he's looking south.
Florges is a fully evolved Flabébé and Floette that boasts amazing Special Defense and high Special Attack. Minty's Florges can use Moonblast to deal STAB damage, Petal Dance to surprise Water-type foes etc., Misty Terrain to protect itself from status conditions, and Aromatherapy to cure status.
Follow the path all the way south from Minty to reach the main entrance of this cavern, from the perspective of Sun and Moon anyway. Past here, you'll arrive at the far end of the landslide-hit path running along the south-eastern side. To the right, there's a hiker stretching side-to-side, facing away from the mountain.
Gigalith is a fully evolved Roggenrola and Boldore with extremely high Attack and Defense, offset by its extremely poor Speed. For attacks, it can use Stone Edge to deal incredible STAB damage (especially if a critical hit lands), Sandstorm to trigger a sandstorm, and Stealth Rock to damage switch-ins. Beware of Sturdy.
Further south is a thick patch of tall grass. Wade through it, towards the south-east corner, to find an invaluable PP Max sitting outside of the grass. If you've got no other Pokémon to use this on, you can use it to boost Necrozma's Photon Geyser from 5 PP to 8 PP. Now retrace your steps until you're back at the icy rock, then head due north.
Ace Trainer Jada
Vanilluxe is a fully evolved Vanillite and Vanillish that has high Special Attack and decent bulk. This one can use Ice Beam to deal STAB damage, Hail to change the weather to hail, Acid Armor to boost its Defense, and Mirror Coat to counter special attackers. Physical attackers are recommended.
Meanwhile, Mismagius is an evolved Misdreavus with respectable special-based stats and Speed. Jada's Mismagius can use Psywave to deal damage equal to the user's Level give or take 50%, and Spite to reduce the PP of its opponent's last move. Note that it has the Levitate Ability, making it immune to Ground.
Past Jada, there's a pair of trainer standing right before the northern exit.
Master & Apprentice Breon and Kaimana
Breon has three Pokémon. His first, Glalie, has the textbook definition of "well-balanced stats". It can use Blizzard for STAB damage, Freeze Dry to damage Water-types, Ice Shard to attack first and Dark Pulse to potentially cause flinch. As with other Ice-types, Fire, Rock, Fighting and Steel-types are recommended.
Next up may be Bisharp, an evolved Pawniard with impressive Attack and Defense. This one can use Night Slash and Iron Head to deal STAB damage, Aerial Ace to combat Fighting-types, and X-Scissor just in case. Fighting-types are ideal, as their attacks do 4x damage, while Fire and Ground are also great.
Finally, Tyranitar is a fully evolved Larvitar and Pupitar with high Attack and otherwise above average stats, sans its low Speed. It can use Stone Edge and Crunch for STAB damage, Earthquake to hit common Types, and causes a sandstorm when it enters battle. It's weak to Grass, Water, Steel and Fairy, and doubly weak to Fighting.
Kaimana has two Pokémon. His first, Vikavolt, is a fully evolved Grubbin and Charjabug with incredible Special Attack, but terrible Speed. It can use Bug Buzz for STAB, Zap Cannon to cause damage and paralysis if it hits, Dig to annoy Rock-types, and Agility to raise its Speed. Fire-types are ideal. Keep in mind it has Levitate.
Forretress is an evolved Pineco with humongous Defense and decent Attack, but low Speed. This one can use Gyro Ball for STAB damage, Double Edge to deal with resistant foes, Iron Defense to raise its Defense, and Explosion as a last resort. It's doubly weak to Fire, and takes neutral or half damage from other Types.
Finally, proceed north towards the exit. On the other side, you'll arrive at a wide open space with a stair lift visible in the far distance. Make your north, towards the patch of tall grass near the center. Step into the tall grass from its southern side, then head west, where a Max Revive can be found in the corner devoid of grass.
After that, step onto the stair lift and use it to transport yourself to the summit of the mountain.
Mount Lanakila (Summit)
Up on the summit, you'll arrive in front of the Pokémon Center. If you helped the Alolan Vulpix in Paniola Town and on Route 8, you can play with it towards the east side. Inside the Pokémon Center, we strongly recommend stocking up on healing items since the Pokémon League is literally just around the corner.
Of note, the right Poké Mart clerk is selling Battle Items. Although they're never necessary for winning a battle, they could be useful to have in a pinch. Especially X Defense and X Sp. Def, to boost your Pokémon's durability. Meanwhile, the rich lady towards the east is the very, very useful Move Reminder .
In exchange for a Heart Scale, she will help your Pokémon learn any move that they've forgotten. That's not even the half of it. Your Pokémon can even learn moves they've never learned or have yet to learn. There's a catch though: Pokémon cannot learn moves exclusive to a previous form. Otherwise, this is one heck of a service!
If you're looking for Heart Scales, you can use Thief on wild Luvdisc, which have a 50% chance of holding one. Otherwise, you can get them by purchasing meals from the restaurants in Konikoni City, Malie City and Seafolk Village. Each day, if you pick the most expensive meal, you'll get double the Heart Scales and not pay a dime!
Outside, start making your way west. After going up the slope to the left of the Pokémon Center, there's a veteran standing in the snow who will only battle you after you've defeated all the "boss" trainers in Alola. So he's the "final boss" trainer. If you've beaten all of them up to now, only the Poni Plains one remains.
Further west, there's a slope leading north. Summon Tauros and charge all the way north through the crystal-lined path. If you're camera crazy, there's a Photo Spot where there's a gap in the east side of the path, near the beginning of the path. At the end of the path, you'll bump into one very inappropriately dressed Professor.
Once you've regained control, step forward and approach open door being guarded by two Ace Trainers. You know what lies on the other side. If you are sure you're ready, step through the door.
Point of No Return
After entering the Pokémon League, you cannot leave until you've conquered all the trainers inside or you're defeated. Although you'll want to avoid the latter since it wastes time and money. So triple check that you've got all the right Pokémon, enough healing items, etc.