From Bulbapedia, the community-driven Pokémon encyclopedia.
Now - FINALLY, everything to do here is done. So, go into the left room of the lab and go through the "hole" in the south of the room. There you will see this map:
Go to the building in the middle of the region that looks like a Poké Ball. This is the Pokémon Dome. If you read the books in the bookcases, you'll learn more about the Grand Masters:
- Courtney, the "Fire Queen", puts her pride in the Moltres who lives in the Legendary Moltres Deck. It puts Energies into Courtney's hand with its Pokémon Power, Firegiver. And its attack, Dive Bomb, is very powerful!
- Steve, the Lightning-type user, can shock you when he puts Zapdos on his Bench because its Pokémon Power, Peal of Thunder. Its attack, Big Thunder, does a great amount of damage! Steve uses the Legendary Zapdos Deck.
- Jack is called the "Ice-Man" for his Ice-type deck, Legendary Articuno Deck. Jack's Legendary Pokémon is Articuno. Its Pokémon Power, Quickfreeze, can Paralyze the Active Pokémon when Articuno is played onto the Bench. Articuno's attack, Ice Breath, should be avoided!
- Rod is the leader of the Grand Masters! He uses the Legendary Dragonite Deck, and he uses Pokémon, powerful as dragons, like Dragonite. Dragonite's Pokémon Power is Healing Wind, which heals each of Rod's Pokémon when Dragonite is played! Its attack is Slam, a powerful move!
The Grand Masters can only be challenged when you have 8 Master Medals.
It's time to go to the Grass Club. (This place is awesome for Fire-types, so it'll be pretty easy if you chose the Charmander & Friends Deck.
Now, let's go to the Grass Club. Go to the south "hole". Then press →, then ↓. In the Grass Club, in comes Ronald, your rival (though he was not introduced at the start of the game). His words imply that he knows you and that he has just won the Grass Medal. He also says that he wants the Legendary Pokémon Cards. Then he leaves. Anyway, go into the lobby at the left. The Lass in the top of the room wants a Oddish, and she'll trade a Vileplume for it. She doesn't care what kind of Oddish you trade her. The girl at the table is Brittany. She is a Club Member, so duel her*. She uses the Etc. Deck, which has types and 2 , , , and two types. But don't worry... She doesn't have any evolution cards! But one thing... She says that the Club Master of the Grass Club, Nikki, is out. Brittany will give you two Mystery booster packs as prizes. Now read "Mail 2" on the PC. But you aren't here to chat, so exit the lobby and go to the main room in the north. Talk to the girl that you see when you enter. Her nam is Heather, and she is a Club Member. She says that she'll tell you where Nikki is if you defeat all 3 Club Members. So, duel her! She uses the Kaleidoscope Deck, which consists of 2 , , , and 2 types, and 3 types. One thing, though —— at least two Pokémon of hers will not be able to be used. The reward for winning is 2 Colosseum booster packs. Now go duel Kristin, the girl on the left side of the room. She uses the Flower Garden Deck, which consists of mostly types, but has two Pokémon, both Lickitung. In other words, a good Fire-type should turn the Flower Garden into a good forest fire. The prize for winning is two Evolution booster packs. ... And the name of where Nikki is —— Ishihara's house! Go to the outside, and press ←, ←, ↑, and ←. Talk to the woman at the desk. That's Nikki. When you talk to her, she will go back to the Grass Club. Back at the main room of the Grass Club, at the top, is Nikki. Save, then duel her!
So, this info might not be the path to defeating Nikki... the first time. If you fail, try, try again.