Pokemon XY - episode 27 - 21 May, 2014

Upon arriving in the next town, Ash learns that the Kalos Region Champion Diantha will be holding an exhibition match, but he misses out on the sign up deadline. He heads to Professor Sycamore's lab, being tasked to help in his research on Mega Evolution, when he meets Diantha there by chance. She allows him a friendly match, Pikachu against her Gardevoir, and she shows she knows what Mega Evolution is. (Source: Wikipedia)