First, go to the Rito Cave and get the Delivery Bag from the Rito there. Then, go upstairs and talk to Medli, who will give you the Father's Letter. Show this letter to Komali (on the bottom floor in the room with green and orange triangles) and then go through the tunnel on the first floor. Throw Medli to the other side of the area you're in when the wind is blowing in that direction, and she'll give you an empty bottle. Fill the bottle with water on the bottom area, and use the water on one of the dry bomb flowers. Use the bomb flower that blooms to blow up the rock, filling up the area with water. Swim to the other side, and throw bomb flowers into the pots there to bring them down. Jump across these platforms to reach Dragon Roost Cavern.
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You have to reach the peak of Dragon Roost and rescue Medli.
Salamence learns Roost via TM. If you have used up your Roost TM, you can breed a male Pokemon of the Dragon Egg group that knows it to a female Bagon, Shelgon, or Salamence.
Fire Mountain is one square south of Dragon Roost Island. You need ice arrows to enter.
No they cant even learn roost.
Unfortunately, the move Roost did not exist in FireRed/LeafGreen, or at all in Gen III. It was introduced in Gen IV. The only way to get Roost on a Crobat is to use TM-51 in Gen IV.