You dont have to be in 4th gear...
See, this is another reason why I hate the guides for these games, this part of the game is completely no existent in the guides. No map of the area, nothing, nadda.
Anyway, I did this a few hours ago acutally.
You have to have the National DEX first of all. The guy...yes, he looks alot like a girl, and I still say he is a girl. Will team with you almost immediantly after you enter the Mountain. There is this small area u have to Rock Smash/Strength your way though first. WHich btw, MAKE SURE YOU HAVE THESE MOVES ON YOU!!! You dont wanna make your way back here like I did many times. It's annoying.
Find the routes that have ash? falling from the sky, and keep moving north. You might find a dead end of sorts, but looks closely. There are ramp rocks close by. Use ur bike to ramp them, and keep moving north. Just keep going and you will see stairs, climb, and the Mountain Entrance is clearly visable.
After you meet up with Buck, you have to make your way through the mountain, and if you dont have repels, get prepared for ALOT of battling >.<. Macargo, Golbat, and Graveler's. Buck uses a Claydol (thats it). Once you make ur way to the end of the cave, you enter a small room that just reeks of "Legendary Placement". Well, Heatran isnt there...yet. Buck steals this stone thing, and leaves. You must then track him down to the Survial Area. You can reach it by leaving the way you entered the Mountain, and going WEST. You will have to ROCK CLIMB often, and keep moving WEST until you reach the Survival Area. He is in the farthest house on the left I believe. Just talk to him, and he takes it back to the mountain.
Now, go back to the Mountain, and to the small room that Buck wanted to go to earler when he stole that stone thing. Heatran will be there now. Lv. 70, so be prepared. I found Gastrodon to be the perfect counter to it. None of his attacks are efffective against it, and if you have BODY SLAM, you are guaranteed to Paralyze Heatran if you are at least lv 45+. If you dont have one, then Emploeon would be awesome. He uses FiRE and Steel attacks.
Also, Heatran is the first Legendary to have a 50/50 chance of being MALE or FEMALE. So you can decide which gender you want if you care by resetting the game AFTER U SAVED IN FRONT OF IT BEFORE BATTLING>
Hope that helps.