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The secret is a wild Heatran, a Fire/Steel-type Legendary. Heatran can be male or female, but is still unable to breed. It is at level 70, and is quite a powerful foe knowing the powerful move "Lava Plume."

Heatran is slow, but still has amazing attack power. It is best to weaken him/her to red health zone and put it sleep. A move like Surf would be best to use if your Pokemon isn't too strong. Like with all legendaries (and any other Pokemon, really) you should keep throwing Ultra Balls until it is caught. Dusk Balls also work since this battle is in a cave, and even more better if it is dark out.

Also because of Heatran's high attack power, brining around a couple revives would be a smart idea.