Instructions
Complete any Wonder Mail missions you receive as you progress through the main storyline of "Explorers of Time." Wonder Mail missions are randomly generated and usually require you to perform a specific task, such as rescuing a Pokemon or battling a particular foe.
Obtain the Mystery Part item. This item is randomly rewarded to you after completing a Wonder Mail mission.
Travel to Mt. Mistral. You cannot access this hidden zone until you accept any mission with an unknown location. If the mission location shows up as "???," that mission will take you to Mt. Mistral.
Fight your way through the first 18 floors of Mt. Mistral. When you reach the 19th floor, you will encounter Ho-Oh, as long as you have the Mystery Part in your inventory.
Place a Water, Ground or Rock-type Pokemon at the head of your battle party. Ho-Oh is a Fire and Flying-type hybrid, making him weak to those particular elements. Ho-Oh spawns at level 43, so Pokemon at or above that level will be sufficient.
Attack Ho-Oh with elemental abilities until he is defeated. At this point, he will ask to join your rescue team. You cannot capture Pokemon in "Mystery Dungeon" games; you can only add them to your team at their request.