How to Unlock Heatran
The same as every other Legendary in the game, you can only access Heatran once you’ve completed all of the Main Missions and let the credits roll. Unlike some other Legendaries, you do not need a full Pokédex to go after this one.
Once the credits have rolled, you’ll have to find the familiar face of Volos in the Obsidian Fieldlands and talk to him to activate the quest The Plate of Firespit Island. The name of the quest makes Heatran’s location pretty obvious.
However, you can’t head straight there. Follow the quest through the Cobalt Coastlands and all of the dialogue and cutscenes it entails until it eventually sends you to Firepit Island to face off with Heatran itself in the Altar of the Cauldron. Following the quest markers is easy, so you shouldn’t have much trouble with the actual location of Heatran.
How to Catch Heatran
When you first come up against Heatran, it will be surrounded with a Fire Shield. To break this down, throw Balls of Mud repeatedly to stun it and the shield will eventually drop. Once you’ve done this, it’s time to send out a Pokémon and do battle.
Heatran is a / dual-type Pokémon. It will be Level 70 when you come across it, so prepare for a pretty tough fight — though at this point in the game, your Pokémon should be sufficiently leveled enough to take it on.
Because of its typing, Heatran is vulnerable to Fighting, Water, and Ground-type moves. Doing four times the damage, Ground-type moves are particularly effective, so bear that in mind.
Due to that dual type, it’s strong against a variety of Pokémon and tougher than many Legendaries. Normal, Flying, Grass, Bug, Psychic, Ice, Dragon, Poison, and Steel-type moves will all do far less damage to it, so pick your partners carefully.
However, the last thing you want to do is make Heatran faint, as then you won’t be able to catch it. Whittling down the health to the yellow (or preferably red) bar is ideal and a status move to paraylyze or make it drowsy is even better, as this will make catching easier. Poisoning and burning may be a bad idea, however, as its health may go down so far after every catching attempt that it eventually faints.
Due to its high level, Great Balls may not do the trick and Poké Balls certainly won’t. Have plenty of Ultra Balls on hand so you can continuously throw them at Heatran until it eventually succumbs.
After defeating Heatran, you’ll have to sit through a cutscene with Iscan and Irida. You’ll be given an** Iron Plate** as a reward, and the mission The Plate of Firepit Island is now complete.