Go to the very left of the Pokémon Village. You must have a Pokémon that knows Surf. Follow the stream up to the waterfall and hop out to the right. Walk right and Mewtwo's cave will be there. You have to have defeated the Pokémon League to enter it.
*Mew is not catchable in Pokemon x and y, but you can get him online from wondertrades and such. Mewtwo however, is catchable and you can find him in the cave in pokemon village.
The Mewtwonite X is exclusively for Pokémon X while the Mewtwonite Y is exclusively for Pokémon Y, so in order to get both Mega Stones (or more of the same one) and another Mewtwo in a single game, trading is required.
Yes You Can Get A Pikachu In Pokemon X and Y.
The Legendary Pokemon available in Pokemon X/Y are:Xerneas (X only)Yveltal (Y only)ArticunoZapdosMoltresMewtwoZygarde
Pokemon X and Y are sold at MSRP of $39.99 (CAD)
The Seaside Cave only exists in the Pokémon Black 2 and White 2 games, not in Pokémon X and Y. It basically connects Undella Bay to Route 21.
You can get to Shalour City by heading East of Geosenge Town, the one with the big rocks, and going through the Reflection Cave.
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Prof. Sycamore will give you the oval charm in Pokemon X and Y.
Pokemon capable of learning Psywave as an Egg Move in Pokemon X and Y are Latios (Pokemon) and Gastly (Pokemon).
I believe X is stronger, but Y looks cooler.
The MSRP of Pokemon Y or X is $39.99 (CAD/USD)