Pokémon Mystery Dungeon: Blue Rescue Team and Red Rescue Team/Fiery Field

After obtaining Surf and receiving the Clear Wing from Spinda, go through the great canyon to Xatu, who will tell you to go to fiery fields to find information on the mirage Pokémon. Now begins your quest for this Mysterious Pokémon. To enter Fiery Field, you will need the HM surf. You will also need to quick save.

The Dungeon
Fire Pokémon have regained their old glory in this game, and the field unleashes it all. Except for the random Dittos, Ursaring, and Machoke all of the Pokémon here are of the fire type.

Items

 * Fire Stone (randomly on floor 29)
 * Lots of orbs, mainly escape orbs

Boss Battle: Entei
On the 30th floor you will fight Entei. Once you defeat him, he will turn your clear wing into the red feather, and grant you access to Lightning Field, which is considerably easier than this.

Entei can only be recruited later after beating the game.

Entei has a size of one star, so after recruiting him, he is powerful and small, unlike other legendaries (4 stars). That is why you can usually have only one legendary on the team when completing dungeons.