Instructions
Travel to Stark Mountain. Stark Mountain is located on an island in the northeastern corner of Sinnoh. Go east on Route 226 from the Survival Area and north on Route 227.
Ascend the mountain and enter its interior caverns. Travel through the caverns, using the Rock Smash move to destroy rocks and Strength to move boulders obstructing the path. If you do not have Pokemon with the Rock Smash and Strength abilities in your party, travel to a Pokemon Center in any town and withdraw one from storage.
Head toward the north end of the cavern system. On the way, you encounter a trainer named Buck, who temporarily joins your party. Enter the small room at the end of the cavern path and wait for Buck to take the Magma Stone. Exit the cavern the way you came.
Return to the Survival Area southwest of Stark Mountain and enter Buck's house. Speak to Buck and return to Stark Mountain. Enter the mountain and return to the room with the Magma Stone. A Level 70 Heatran appears when you enter the room.
Capture Heatran. Begin the battle and select "Item" from your action menu. Select a Pokeball from your inventory to attempt to capture the Heatran. Legendary Pokemon generally have low catch rates, so you may need a large supply of Pokeballs. Repeat this action until you have captured Heatran.