How far into the story do you have to be to fight the dragon on Mount Houzell?
Viewing post in Merged World, need help.
If you reach the point at which you can even visit Mount Houzell in the first place (after having retrieved the airship in the game's second part), there's not much to do anymore. You need to advance the story at the Rift Shrine so that you can explore all five locations where Elemental Fiends reside (Mount Houzell is one of them), and you need to defeat Bahamut so that the dragon at Mount Houzell spawns.