Enter all of the houses, and you'll eventually find Buck and his grandfather. First, talk to Buck's grandfather and he'll tell you that the legendary Pokémon Heatran will be enraged if the Magma Stone is taken, and Stark Mountain will erupt! Next, talk to Buck. He finally knows that this is a serious matter and bolts off to put the Magma Stone back in its proper place.
Make sure that you talk to Buck and his grandfather, heal your Pokémon and buy at least 40 Dusk Balls before proceeding back to the mountain! Be aware that you'll be facing a Level 70 Legendary Pokémon. You will also need the HMs Surf, Rock Climb, Strength, and Rock Smash. Also, bring a Pokémon with Cut for the next step of the journey.
Back to Routes 226-227
Make sure that you talk to Buck and his grandfather, heal your Pokémon and buy at least 40 Dusk Balls before proceeding!
Go through Routes 226-227 again.
Back to Stark Mountain
It should be easier to go through the volcano this time because you know where you're going.
Go through the room as you did previously.
Now that you're going through this room on your own, you're free to use Rock Climb now, so you should pick up all items in this room.
Buck is nowhere to be seen, but a Pokémon is standing in front of you. That is the legendary Pokémon Heatran. Heatran looks angry...... no, it looks furious. With Heatran enraged, the volcano will erupt for sure and destroy everything on Battle Zone. Seems like it is too late......
Wait, there is still one way to solve this mess and prevent the volcano from erupting. Guess what? You'll need to catch it. Another legendary Pokémon for you to catch!
When you're ready, talk to Heatran, and the battle will start. Heatran is a Fire/Steel-type Pokémon, and is at Level 70. Ground-type moves deal significant damage to it, so be careful if you're unleashing something like Earthquake or Earth Power. Start off with moves that causes sleep or paralysis (do not bother with freeze— it will instantly defrost with Fire-type moves. Heatran is also immune to being burned and poisoned). Follow up with a Water- or Fighting-type move. Then, keep on damaging with weaker attacks until its HP bar is red. After that, keep throwing Dusk Balls (as you're in a cave, these work best) until you successfully catch it. It does not know its signature move, Magma Storm, but watch out for Lava Plume, which can still hurt a lot! Iron Head can also potentially hit you hard. Do not hesitate to heal your Pokémon often.
Once you've caught Heatran, get yourself out of the volcano and continue to Route 228. From the T-junction of Routes 226 and 228, head east into the gate. Or, if you're not ready, Fly back to Survival Area first and heal up.
Make sure that you heal your Pokémon and buy a lot of medicines before proceeding! You'll need to bring Pokémon with Cut, Surf and Rock Smash.
You'll want to keep heading south to get to Route 229, but try to explore all over to get all of the items (a lot of which are rare or valuable) and fight all of the Trainers. Don't hesitate to heal if you need to.