Where do you go after you go to the volcano in Pokemon diamond?

If you have already gone to the end of the volcano and back then you have to go to the survival arena and talk to buck. He will put the magma stone back and you go back there to fight the level 80 heatran. Fire and steel type, moves: fire spin, iron head, ancient power, and lava plume. bring a water, or ground type or anything that is effective.