Instructions
Raise your Pokemon to at least level 40, and make sure at least one of them knows a reliable ice-type attack such as Icy Wind, Ice Punch, or Ice Beam. Buy several Hyper Potions at the Poke Mart and enter the Blackthorn Gym.
Use Strength to push the boulders into the lava and defeat the gym trainers. Make your way across the lava path to reach Clair.
Defeat all three of Clair's level 37 Dragonairs with ice-type attacks. Lapras's Ice Beam and Feraligatr's Ice Punch work very well. Heal your Pokemon if necessary.
Inflict Clair's Kingdra with a status condition, preferably Sleep or Paralysis. Use powerful moves such as Body Slam and Ice Beam to whittle down Kingdra's HP. Heal your Pokemon if necessary and continue to attack until Kingdra is defeated.