The Heatran quest is, with the Jirachi quest, currently the only quest which requires a region specific play time requirement (150 Hours). Other than a usual hour requirement, the hours needed for this quest will be counted from the specific beginning time of the Sinnoh region. This time can be checked in Professor Rowans lab in Sandgem Town after finishing the story part of this region.
The Heatran quest is the only quest which gives the player a option to choose two different ways to encounter the legendary Pokémon.
It is recommended to prepare well for this quest as you have to fight trough a large area with many trainers in order to be able to catch Heatran. Entering this area requires a Pokémon with a level of 60 or less. Therefore a good choice of your partner is important.
Depending on which way you choose to catch Heatran, following Pokémon are often recommended from players:
By choosing a Pokémon, you should keep in mind that all Rare Candy's and Master Balls will be taken away from a NPC for the duration of time the quest lasts.
Stark Mountain
The closest Pokémon Center to Stark Mountain is located at Fight Area, therefore you have to pass trough Route 225, Route 226 and Route 227 in order to enter Stark Mountain and fight trough Start Mountain Entrance Pt to reach Buck and the starting point of the quest. Based on this fact, it is recommended to clear this areas first with a team of high level Pokémon as Stark Mountain can only be accessed with one single Pokémon in your team which has a level of 60 or less. Passing through this area without clearing it first, would result in not wanted level ups regarding this mentioned Pokémon and level requirement.
Once arrived at Stark Mountain, you can enter the cave, pass through the first part, and talk with Buck. When you hit the requirement of having only one Pokémon in your team which is level 60 or less, the quests starts. Buck will take all Rare Candy and Master Balls from you, you can also not leave this area without canceling the quest. Once left, you need the same requirements to enter the Mountain. Same counts for if you lose to a trainer in there.
Once you are in the Mountain, you have to cross through it in order to enter the inner cave where Heatran is located. For that, you have to pass through many trainers. As help, you can catch some Pokémon in the cave to be able to heal up your main Pokémon with Revives or similar items, in case it gets defeated. Doing all trainer can be a advantage in such a way, that the final fight, which must be done after this part of the cave, could be more easy. Anyway, the shortest, that means the way with the fewest trainers, is shown below.
All NPCs cool down after 7 days unless noted contrariwise. All NPCs may have new Pokémons, or higher level Pokémons upon rematch unless noted contrariwise.
Stark Mountain Heatran Pt
Once you have reached Heatran's room, you will have a dialogue scene and have to fight Commander Mars.
Emboldened moves avail from a STAB bonus when deployed by that Pokémon.
Italicized moves are functionally broken; see their individualized pages for more information.
Once beaten, you have two different options to carry on.
Now, as Commander Mars is beaten, you can fight Heatran which is on level 99 and have the now the chance to catch it. However, you can also defeat it in order to talk to Buck who will give you your Rare Candy and Mater Balls back and you can leave the cave and re-enter it without any requirement. If you choose to defeat it, Heatran will re-spawn in his room but is level 30 this time.
In case Heatran was not successfully caught, you have the chance to catch it again by beating Nikola.