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Leganderies:

#716 Xerneas & #717 Yveltal;

Xerneas & Yveltal are found during the end of the game in Team Flares HQ. Both poekon are set captures so they will only need one pokeball.


#718 Zygarde;

After beating the elite four, you can find Zygarde deep in Terminus Cave. Regardless to what you might think you do not need any TM's or HM's.


#150 Mewtwo;

After defeating the elite four, you can find Mewtwo in pokemon village. You will need the HM surf to cross to the other side of pokemon village which is located on the far right side. You enter a cave near the top and Mewtwo will be there waiting with his Mega Stone.
Tip: Bring Dusk Balls as the battle is in a cave so it will increase capture rate.


#144 Articuno, #145 Zapdos, #146 Moltres;

After defeating the elite four you will randomly encounter one of the three legendary birds depending on which starter you picked. Zapdos if you choose Fenekin, Moltres if you choose Froakie and Articuno if you choose Chespin.

Now after you encounter your legendary bird once, it will appear in the pokedex. Knowing this you will follow the bird and you must encounter it 10 more times and then it will fly to Sea Spirits Den where you can finally battle and capture him. ( For the people who don't know, the legendary birds flee upon battle. )

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