Instructions
Go to Blackthorn City after you have gotten the first seven badges and defeated Team Rocket three times -- the third time is when you fight them at Goldenrod City. You can reach Blackthorn City via Route 44 and the Ice Path east of Mahogany Town.
Head to the northern half of the city, where you will find the Blackthorn City Gym. Enter and challenge Clair. Before you can beat her, you need to fight your way through the gym's trainers. Work your way through the gym until you get to the end -- you will need a Pokemon that has the Strength skill so you can move the boulders that block your path.
Fight Clair. She has all Dragon-type Pokemon, so your best bet for this trainer battle is to bring at least one or two Ice-type Pokemon. She has three level 37 Dragonairs and a level 40 Kingdra, so your Pokemon should be at least that level or higher.
Leave the gym after defeating Clair and go to the Pokemon Center. Speak with the woman at the desk and have her heal your Pokemon. Return to the gym, but go around to the back. Surf across the water to reach a cave -- speak with the man out front and enter.
Approach the water and surf across it bearing south -- the water here is full of Dratini, which are fairly strong Dragon-type Pokemon. If you still have your Ice-type Pokemon in your party, you should have no trouble navigating this cave.
Cross the whirlpool and proceed southeast until you reach the far eastern wall. Head north from here to reach a strip of land. Follow the path to the north and examine the item to obtain a Dragon Fang. Clair appears beside you and gives you the eighth gym badge -- the Rising Badge.