Having made your way through Victory Road in Pokémon Brilliant Diamond and Shining Pearl, you'll come to the Pokémon League.
Four tough opponents stand between you and the ultimate battle, the last of which is Lucian.
How to beat Elite Four Lucian in Pokémon Brilliant Diamond and Shining Pearl
When it comes to facing the Elite Four, 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 50.
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.
Psychic-type Pokémon are often very strong, and can usually set up the battlefield to debuff and hinder while buffing themselves. This is no different for Lucian, with the likes of Light Screen and Trick Room used by Mr. Mime, Girafarig, and Bronzong.
Meanwhile, Alakazam is always strong, while Medicham is there to bring the Fighting-type pain.
Make sure you heal up and save your game following the fight, and prepare to face the Champion...