Once you've acquired 7 Gym badges, you'll be able to challenge the final Gym, here at Hammerlocke Stadium. Go over and chat to the staff member at the very end of the corridor to begin the Gym mission. You'll be taken to the vault, which you visited earlier.
Know Your Match-Ups: Dragon
Mystical Dragon-type Pokémon are weak to Ice, Dragon and Fairy. They're strong against Dragon (themselves) and resist Fire, Grass, Water and Electric. However, they do no damage to Fairy-types.
This Gym is unique as only half of the Pokémon are Dragon-type; the other half focus on weather effects. So you'll want some strong general use Pokémon on top of dragon slayers. It also helps a lot if you have Pokémon that can change the weather; you can buy weather TMs from Hammerlocke's western Pokémon Centre.
The mission here is pretty simple, but depending on your experience, it may be challenging still. All you have to do is defeat 3 Gym trainers, with the opportunity to take breaks in-between. But all of the battles will be Double Battles and your opponent will trigger a specific weather effect. Anyway, your first battle will start straight away.
Gym Trainer Sebastian
Pelipper is an evolved Wingull; it takes 4x damage from Electric and 2x damage from Rock. Sliggoo is an evolved Goomy; you can find both in the Lake of Outrage in specific weather conditions in Pokémon Shield.
Sebastian relies on Pelipper summoning rain at the start of battle due to its Drizzle ability. This will power up Pelipper and Sliggoo's Water moves, while allowing Sliggoo to shake off status conditions via its Hydration ability. Pelipper can also use Tailwind to boost its team's speed. So try to zap Pelipper ASAP.
Gym Trainer Camilla
Ninetales is an evolved Vulpix; it's more common in Pokémon Sword. Turtonator can be found in the Lake of Outrage in Sword only; it's weak to Ground, Rock and Dragon.
This time, you'll be blessed with sunny weather thanks to Ninetales's Drought ability. Obviously this will power up Camilla's Fire-type Pokémon, while weakening your Water-types. If possible, use Rain Dance straight away to turn the tables.
Ninetales can also use Will-O-Wisp to burn, while Turtonator can set up a Shell Trap to damage Pokémon that hit it with a physical move. As such, special attackers are highly advised.
Gym Trainer Aria
Abomasnow is an evolved Snover; it takes 4x damage from Fire and 2x damage from Flying, Rock, Poison, Bug, Fighting and Steel. That's a lot of weaknesses! Meanwhile, Hakamo-o is an evolved Jangmo-o; it takes 4x damage from Fairy and 2x damage from Ice, Flying, Psychic and Dragon.
This battle centres around hail, triggered by Abomasnow's Snow Warning. This will let Abomasnow use Aurora Veil, which reduces physical and special damage. Also, Hakamo-o will not take damage from hail because of its Overcoat ability.
Compared to the previous battles, this one has less going for it, in theory. That said, you'll want to quickly KO Abomasnow (Fire moves are perfect for this) before it gets a chance to use Aurora Veil.
Succeed against Raihan's carefully selected elites and he'll welcome you to the stadium grounds.
Gym Leader Raihan
Your battle against Raihan will also be a Double Battle. First, he'll send out Gigalith and Flygon. Gigalith is a fully evolved Roggenrola and has the Sand Stream ability, which creates a sandstorm when it enters battle. Meanwhile, Flygon is a fully evolved Trapinch; it takes 4x damage from Ice and 2x damage from Dragon and Fairy.
If you're uncertain, it might be a good idea to change the weather on your first turn. Rain is a good choice as Gigalith is weak to Water and Flygon does not resist Water. That said, Gigalith may take advantage of the free turn to set up Stealth Rock, which slightly damages Pokémon you switch in.
Next, Raihan will bring in Sandaconda, which is an evolved Silicobra. If you changed the weather, attacking Sandaconda will make the sandstorm come back because of its Sand Spit ability. It can also use Glare to inflict paralysis. So try to remove it from the field, rather than change the weather again. But be wary as it can Protect.
Finally, Raihan has a Duraludon that he can Gigantamax. This new Dragon Pokémon is weak to Ground and Fighting only. Although it has terrible Special Defence, so most special moves will hurt it. Its G-Max Depletion will reduce the PP of its opponent's most recently used move. It can also use Max Rockfall to change the weather to sandstorm.
Survive Raihan's draconic malestrom and he'll reward you with the final Gym badge, the Dragon Badge. Afterwards, you'll be given TM99 Breaking Swipe and a Dragon Uniform. Well, you've done what you came here for; leave the stadium. As you step back into the streets of Hammerlocke, you'll find Magnolia and Sonia again.
Once you're back in control, head for the Hammerlocke train station, towards the east, just before the entrance to Route 7. Inside the train station, Hop will catch up with you. Wow, did he defeat Raihan that fast? Or were you taking too long? Anyway, you'll automatically board the train with Hop. Upon arriving at the next train station, exit to the right to reach Route 10.
Time to Catch 'em All!
Having acquired all 8 Gym badges, you can now catch Pokémon of all Levels. That includes all the super strong Pokémon roaming the Wild Area! This might be the final stretch, but if your party has some gaps or a weak link, it could be a good idea to have a look around the Wild Area for some replacements.
In particular, the Lake of Outrage, west from Hammerlocke Hills is home to many strong and rare Pokémon. That said, even though you can catch them, they're still strong. So exercise caution or you may end up with more than you bargained for!