How Do I Get a Legendary Pokemon?
Pokémon Quest Guide
Pokemon Quest offers a different approach on acquiring new Pokemon to fight by your side. Here, instead of throwing a Pokeball to catch them, you get to lure new Pokemon by preparing irresistible recipes that they'd go after.
To cook up anything, simply check the cooking pot at the center of your Base Camp. Depending on the ingredients that you'll put there, you can create a variety of dishes with different size offerings to gain the different types of Pokemon, in varied levels and random default moves.
Basically, you'll have to do the same to acquire a Legendary Pokemon. To get a random Legendary Pokemon in the game, what you'll have to cook is the dish called "Ambrosia of Legends ala Cube". To successfully cook this recipe, you'll need an ingredient called Mystical Shells. Don't be surprised though if you do not get any of it during your first few levels in the game. You'd get the chance to acquire this as loot -- only from levels 12-1 above. You'll know you'll have it when you get to encounter a Legendary Pokemon for the boss battle.
Ambrosia of Legends ala Cube
Known Recipes:
(1 slot) Mystical Shell + (4 slots) any other ingredient
(3 slots)Mystical Shell + (2 slots) Rainbow Matter
(4-5 slots) Mystical Shell
Will lure a random Legendary Pokemon like:
Articuno, Zapdos, Moltres, Mew, or Mewtwo