Arcade Star Dahlia

Dahlia is the Frontier Brain of the Battle Arcade. You fight her twice, just like all of the others. Upon meeting her she claims that your encounter is just luck, and that she will conquer. Her first set isn't hard, just hit her with the roulette and abuse moves like Hypnosis and Blizzard. For the 2nd fight, she has strong moves. You'll need a bit of luck. Have your Pokemon hold the Focus Sash, or Scope Lens. A riskier tactic is bringing a few Magikarp and getting Pokemon swap.