Instructions
Fight the enemy you see immediately after you begin the level. A few well-placed attacks should take him out easily. Move up the stairs once he is defeated. You will come to a room with several boulders.
Push the boulders into the nearest holes in the floor. Do this for every available boulder except the one on the far top right. Leave this one empty so that you can jump through to continue to meet the gym leader, Claire.
Defeat Claire by primarily using your "Ice Pokemon." She is also susceptible to the "Explosion" attack, but that will drain much experience and can weaken you for the rest of the level. She is protected by several Dragonaires. Use your "Psychic" skill to defeat them and leave Claire unprotected for attack. Once she is defeated, you will move past the Blackthorn City Gym and into the Dragon's Den.