okay, you have to go to stark mountain, you go through stark mountain and you will team up with buck (before you team up with buck you will see you're friend standing by an opening, go through and walk straight on and buck will be there and he says lets team up)
look around for abit and you should find another doorway, buck will leave you and go through follow him, he will take this thing called a magma stone, then he'll leave. either use an escape rope or dig to get out of stark mountain quickly, at bucks house somewhere in one of the area's buck will be in a house. he should say something then in the room where he took the magma stone will be Heatran, battle it :D if you want catch it, it will be at level 70
im unsure what happens at bucks house because im about to go there myself.