Heatran
First of all, you must have beaten the Elite 4 and have the national dex:
1)Head toward Stark Mountain.You'll meet up with your Rival and Crasher Wake.
2)Go further down the path and enter the Mountain
3)There will be 2 Team Galactic grunts,the Galactic Scientist,Mars, and Jupiter
4)Beat them both then the Scientists and Grunts will run further into the cave
5)Follow them then to the next room
6)Buck will appear and follow you them
7)Get through the maze and enter the next room
8)Talk to them and they'll take the Magma stone
9)The Looker will appear from a disguise and his Croagunk will take the Magma Stone
10)Some police officers will appear and take them away
11)Go to the house west of the Pokemon center in the survival area
12)Talk to the old man behind the counter
13)Go back to the last room in Stark Mountain and BAM!!!Heatran
Hope I helped
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