The Turffield Stadium is the first Stadium you’ll actually be able to fight in during your quest through the Galar region. Stadiums are a lot like the Gyms of old, except they’re much larger and more elaborate.
In order to get through each Stadium, you’ll need to first clear the Gym Mission, which is usually in the form of a puzzle where you fight against Trainers along the way, and then you’ll have the privilege of challenging the Gym Leader in a massive Stadium large enough to accommodate Dynamaxed Pokémon.
Once you’ve started the Gym Mission, you’re unable to swap Pokémon out from your PC boxes, and if you leave the Gym Mission before clearing it, you’ll have to start back from the beginning of the Mission.
Grass-type Pokémon are the primary Pokémon type you’ll have to deal with here. Having Fire-, Flying-, or Ice-type Pokémon is a good idea, although Bug- and Poison-type Pokémon will also do well against most of the Grass-types you run into here as well (they aren’t as effective against the several Grass/Poison-types you fight).
Inside of the Stadium, make sure you talk to the Ball Guy. Talk to him and he’ll give you a Friend Ball, which starts the Pokémon caught with a higher friendship stat.
You can buy different uniforms at the shop on the left in the entrance hall to the Stadium. These are complete clothing sets and cost 
18,000 each. You don’t get to wear these during your Gym Challenge, but you can wear them like standard clothes, and you can mix and match pieces or put on the whole outfit at once.
The selection available varies based on which version you’re playing; you’ll be able to buy all of the uniforms for types of Gym Leaders that you do not fight in your version, because you’ll earn uniforms for each type of Gym Leader you defeat, but the Gym Leaders vary by game.
Anyway, make sure you have healing items and your Pokémon are fully healed, then talk to the person towards the back of the lobby and you’ll be given a brief explanation of the Gym Challenges, then will be asked to change into your uniform if you’re ready. Do so and you’ll enter the Gym Challenge!
Gym Challenge
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In order to get through the Turffield Gym Challenge, you’ll need to herd some Wooloo! You have to chase them over to the blue zone to the north.
The first room is very easy. All you need to do is just walk forward and make sure that the Wooloo move forward along with you. If any of them stray off to the sides, just run over towards them and guide them back to the center.
Roll them all onto the blue zone and they’ll break through the hay and leave you free to tackle the next section.
The second section is a little bit more complicated as there is a Yamper running around in the center that will chase away the Wooloo if it sees it, so you’ll want to herd the Wooloo while the Yamper is in the other direction.
It’s very easy to guide them to the left or right and just force them all north regardless of the Yamper, but you can also rush ahead and fight against Gym Trainer Samuel. Any time you beat a Gym Trainer in this challenge you’ll also stop any Yamper from running around, so sometimes it might make more sense to rush to the Gym Trainer.
You have to fight Samuel, but some of the others are optional.
In the next section, there’s a Yamper running in a circle between the left and middle paths, with a Gym Trainer to fight on the right. You can avoid the Gym Trainer by just guiding the Wooloo to the left while the Yamper is running up on the left at the same time. Otherwise you can fight Gym Trainer Mark on the right to stop the Yamper and make it pretty easy.
The final section has two Yamper running along the left and right sides with some hedges in the middle. It may look tricky, but it’s also very easy — just wait until the right-most Yamper is running up on the far-right side and then herd all of the Yamper against the right wall so they’re behind the Yamper. They’ll all funnel in that passage behind the Yamper. You can also fight Gym Trainer Leah beyond the middle hedges to stop the Yamper and get some experience, but she is optional.
After breaking the hay to the north, you’ll unlock the way to challenge the Gym Leader in the stadium’s pitch. Climb up the stairs if you’re ready — or even if you aren’t ready. If you want to heal up your Pokémon, you can leave the Stadium after entering the walkway to the pitch and you’ll be able to resume where you left off.
If you need to leave at any time before that, you’ll have to start the puzzle of the Gym Challenge over, but any Trainers you fought will remain fought along with pacifying the Yamper, so it shouldn’t be too rough to leave to heal up if you need to, but it’s really best to wait until after you’ve cleared the whole Gym Challenge since you can get right back into the Gym Leader fight without clearing the puzzle if you’ve already cleared it.
Gym Leader Battle
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Milo uses two pure-Grass-type Pokémon — a Gossifleur and an Eldegoss — so you’ll have five types super-effective against them: Fire, Flying, Bug, Ice, and Poison. Attacks of any of these types will do well against Milo.
If you started with Scorbunny, it should be a Raboot by this point and should have no trouble with the Gym Leader at all; if you chose Sobble, though, you’re going to want support from another Pokémon like Corvisquire, Growlithe/Arcanine, Vulpix/Ninetales, Dottler, Butterfree, or any other Pokémon that is super-effective against Grass. Having a resistance to Grass-type moves helps, too.
Milo’s Gossifleur and Eldegoss both have Magical Leaf, a 60-power Special-based Grass-type move that ignores accuracy and evasion modifiers. That hits reasonably hard, so don’t underestimate it, although again having a resistance to Grass will be a big help.
While Milo’s Gossifleur shouldn’t be too tough to take down, his Eldegoss will unleash the power of Dynamax on turn 1, transforming into a super-sized Dynamax form for three turns total. In its Dynamax form, it will gain a substantial increase to its HP and damage output. You can choose to either Dynamax your Pokémon on the first turn as well to counter it, or you can choose to focus on lowering its stats for three turns before opting to Dynamax yourself, so you can take it down in its weaker state. Just be sure you don’t waste your Dynamax opportunity against his Gossifleur — you should almost always use Dynamax against the Gym Leader’s final Pokémon, regardless of which Gym Leader you’re facing, rather than one of their earlier Pokémon.
Milo’s Dynamaxed Eldegoss turns its Grass-type moves into Max Overgrowth, which does a lot of Grass-type damage and also sets up a Grassy Terrain which will heal all Pokémon on the ground for a bit of their HP between turns. This also will boost the power of Grass-type moves by 50% while its effect is active, so future hits will do even more damage.
After Beating the Gym Leader
After beating Milo, he’ll bestow upon you the Grass Badge, which allows you to catch Pokémon up to level 25 in the wild.
You’ll be taken back to the lobby and given TM10 Magical Leaf, a decent 60 power Special-based Grass-type move that is unaffected by accuracy or evasion modifiers, meaning it will always hit the foe unless there’s an effect protecting them.
You’ll also be rewarded with the Grass Uniform for defeating Milo. Milo will show up afterwards and tell you that your next stop is Hulbury to fight against the next Gym Leader, Nessa, but you’ll need to go through Route 5 first. He’ll also say that you can return to the Wild Area, which is true and you can catch a few more Pokémon that you weren’t able to before, but it’s probably best to wait until after defeating Nessa or the third Gym Leader before returning — although having more Pokémon options for your team isn’t a bad thing.
Before you leave, you may also want to talk to the boy in the middle of the lobby. He’ll offer to trade you a regular Meowth for a Galarian Meowth. You can catch Galarian Meowth easily on Route 4 but the normal version of Meowth cannot be found in the wild, plus you’ll need its evolution, Persian, in order to complete your Pokédex, so you should definitely trade him. The Meowth is nicknamed Cash and its OT is Mattia. It’s holding a Sitrus Berry and has Technician for its ability.
Whenever you’re ready, head on over to the eastern part of Turffield and you’ll be free to cross the bridge to Route 5.