The first time you enter Wyndon Stadium, go and chat to the Ball Guy on the left. You'll receive a Dream Ball, which is a rare ball with a higher catch rate against sleeping Pokémon. Originally, it was exclusive to the Dream World in Generation 5. Feel free to scope out the rest of the lobby. Otherwise, speak to the receptionist, where Hop is, to sign up.
You'll be taken to the locker room. As soon as you're ready, go through the door straight ahead to reach the entrance tunnel. Then proceed to the stadium grounds to meet your first opponent. You may have fought her fairly recently, near Spikemuth, but this is the first time you're facing off inside a stadium!
Pokémon Trainer Marnie
Following the footsteps of her brother, Piers, Marnie is an expert of Dark-type Pokémon. Her team is the same as before, but with one new addition. Fairy and/or Fighting-types are highly recommended, while a Psychic, Flying and Steel-types would be useful for backup.
Marnie will lead with Liepard. This tricky feline can use Fake Out to cause a flinch on turn 1, Nasty Plot to greatly boost its Special Attack and Snarl to lower your Special Attack. Stat-wise, it's fast but has awful defenses, so even non super-effective moves will cause it severe pain.
Next, she may send out Toxicroak, especially if you used a Fairy. This evolved Croagunk is weak to Psychic (4x damage), Flying and Ground. Given the chance, it can use Sucker Punch to strike first if you selected an attacking move, Toxic to cause deadly poison, Swagger to confuse and Venoshock to hurt poisoned foes.
She can also send out Scrafty. It's weak to Fairy (4x damage), Fighting and Flying. Marnie's Scrafty can use Swagger just like Toxicroak, Brick Break to remove barriers and Scary Face to lower Speed.
Morpeko is Marnie's signature Pokémon, although it's hit or miss. As a reminder, it's weak to Ground, Fighting and Fairy. Each turn, it's swaps forms, but it's pointless right now. For attacks, it has Bullet Seed to annoy Grass-types and mediocre STAB moves in Spark and Bite.
Finally, Marnie has a Grimmsnarl, a fully evolved Impidimp, which she will proudly Gigantamax. As usual, it's weak to Poison, Steel and Fairy. Its G-Max Snooze deals Dark-type damage and causes your Pokémon to sleep on the next turn. It can also use Max Starfall (Fairy). Try to defeat it within 1 or 2 turns!
After the battle, you'll return to the locker room. Once you're back in control, go through the door again to meet your final opponent of the semifinals. Your Pokémon will already be healed, but feel free to change your lead Pokémon. You're also allowed to access your boxes, if you need to shuffle your party.
Pokémon Trainer Hop
|Inteleon (if you picked Grookey)||49||Water|
|Rillaboom (if you picked Scorbunny)||49||Grass|
|Cinderace (if you picked Sobble)||49||Fire|
Hop has the same team as your last encounter, but his Pokémon have a few more coverage moves. All you really need is a good Fighting-type, a Fire or Electric-type, something to deal with Pincurchin and your starter to waste his starter.
First, he'll send out his first Pokémon partner, Dubwool. This time, it can use Cotton Guard to drastically boost its Defence, Reversal to punish Rock and Steel-types and Zen Headbutt to smack Fighting-types. Special attackers are recommended; otherwise go with Fighting. Fire could also work because of Fluffy.
His second 'mon is Corviknight. If you've forgotten, it's weak to Fire and Electric only. On top of the expected STAB moves, it can use Scary Face to lower your Speed and Swagger to cause confusion. If possible, use a special attacker because it has lower Special Defence.
Pincurchin shouldn't frighten you, but it can take a while to whittle down. It can use Poison Jab to poke Grass-types and Curse to boost its Attack and Defence, while lowering its (already low) Speed. A Ground-type would be perfect here, but anything not weak to Electric that can deal neutral damage should be OK.
Snorlax has no STAB moves--only coverage moves. It can use Heavy Slam (Steel), High Horsepower (Ground) and Hammer Arm (Fighting). Use Fighting-types if possible, but otherwise anything's fine, so long as they're not weak (so avoid Normal, Fire, Electric, Rock, Ice, Poison, Dark, Steel or Fairy).
Finally, Hop will send out his fully evolved starter and, because there's enough space, he will also Dynamax it. Obviously this is your cue to send out your starter. Note that Inteleon has Icy Wind (good against Grass) and Rillaboom has High Horsepower (good against Fire), but you'll probably just one-shot them.
Once you've ended Hop's dreams, you'll return to the lobby. Afterwards, you'll head to The Rose of the Rondelands with Hop to wait for Leon. However, it eventually becomes apparent that Leon's a no show. Piers will then arrive with some interesting intel. Apparently, he saw Leon on his way to Rose Tower, but you need to ride a special monorail to get there.