Instructions
Access Stark Mountain. In the Sinnoh region, Stark Mountain is at the very end of Route 227, located on the Northeast potion of the map, past the Survival Area and the Resort Area. After crossing the entrance to Stark Mountain, players will encounter a Pokemon trainer named Buck.
Talk to Buck to learn about the events that have just transpired in Stark Mountain. Apparently, the mountain contains the powerful Magma Stone that many powerful trainers (including Buck) are attempting to find. Enter the mountain and begin to follow the trail.
Reach the top of Stark Mountain by following the mountain's predetermined trail. When prompted, defeat wild Pokemon in battle. In the middle of the journey, Buck will accompany you due to nearly losing in a battle. With Buck following you, continue to the top of the mountain. When required, use "Strength" to move large boulders and "Rock Smash" to destroy smaller rocks.
View the top room of Stark Mountain, which contains the Magma Rock. Buck will leave your side, steal the Magma Stone and run out. Exit out of the mountain by retracing your path, or using the Escape Rope item.
Access the Survival Area that is Southwest of Stark Mountain and Route 227. Visit the houses in the area until you find Buck once again. Talk to Buck, who will reveal that the mountain has been in turmoil ever since he removed the Magma Stone. After finishing the conversation, reenter the mountain and reach the top once again.
Approach the mysterious Pokemon and press "A." You will initiate a battle against the legendary Pokemon Heatran. Battle him with several strong Pokemon, as Heatran is at level 70 during the battle. Weaken him with attacks to reduce his Hit Points (HP) and also inflict statuses such as sleep and paralysis. Once Heatran is weakened to the lowest amount of HP possible without knocking it out, throw a special Pokeball (such as dusk ball, timer ball or ultra ball) to capture it.