To start the latios latios quest you must need 300 evolved dex and complete the jirachi quest
Read more here
https://pokemonrevolution.net/forum/topic/163727-legendary-megathread-complete-quests-and-seen-data/#comment-442665
Does anyone know where this place is, I think I got a different clue from the people I saw from their videos or threads.
I hope someone or mods can help me solve this