Here is a fact - without trading transfering or getting events, It's impossible to have a full team of legendarys in X and y (there is only 4 in one run (one of the legendary birds zap if fennekin arti if chespin or moltres of froakie (the starerts are what you pick at the START of the game (because you get the one thatis weak to your starter's type after trading pokemon with the girl (IDk her name) That is weak to your starter.) they are roaming till they are encountered 10 times then they retreat to sea spirit's den) Mewtwo is found in Pokemon village's cave and Zygarde in terminus cave ( oh the mascots are caught during story) Credits to bulbapedia.