Latias Comes to the Legendary Arena
Also in late March, Latias will arrive in the Legendary Arena. Dragon- and Fairy-type moves are super effective against Latias, so Iris (Champion) & Hydreigon and Diantha & Gardevoir will pack extra punch in this battle. Strategy tips will be available when the arena opens, so give them a look if you find the battle challenging.
Source: Official Pokémon Masters EX site