Having made your way through Victory Road in Pokémon Brilliant Diamond and Shining Pearl, you'll come to the Pokémon League.
After you've defeated both your Rival, and a set of extremely tough opponents in the Elite Four, you'll come up against your greatest challenge yet.
Get ready to face Cynthia, the Pokémon League Champion!
How to beat Cynthia in Pokémon Brilliant Diamond and Shining Pearl
When it comes to facing the Elite Four and Cynthia, you're going to have to use your full team. It's a big step up from the Gyms, which you can easily stomp, and Elite Four Pokémon are all going to be above level 60 - the highest you've faced yet.
Also, she's the first serious trainer you'll encounter who uses a full team of six Pokémon - and she's not afraid to switch them out mid-battle or use healing items.
As such, it's worth taking the time to train your Pokémon, teach them moves via TM, and generally try to have a spread of types in your party.
In short, all of her Pokémon are strong for different reasons. Spiritomb is a slippery Ghost-type with devastating moves, while Roserade has varied, status-and-debuff-inducing attacks.
Gastrodon and Lucario are heavy hitters, while Garchomp can rip through your team. To top things off, Milotic can draw out rounds through self-heals, meaning you need to manage your PP well.
After an epic battle, take stock and save your game.
Congratulations, you are now the Sinnoh Pokémon League Champion!