The Champion Stadium: Unova Challenge is now live in Pokémon Masters EX! You can battle either Champion Alder or Champion Iris depending on the week, so be sure to take them both on!

Champion Stadium Unova Challenge Under Way!

Champion Stadium Under Way! The Champion Stadium Unova Challenge is under way! Defeat powerful opponents for a chance to enter the Hall of Fame!

Unova Challenge

Important

• You can participate in the Champion Stadium after you complete Main Story Chapter 10: A Reason to Battle (Normal) in single-player and collect all PML Badges.
• Medal acquisition and Hall of Fame information will be saved for each region regardless of event date/time.
• Details about the time period and featured region will be included in notifications going forward.
• The time period and featured content are subject to change without notice.

Source: https://pokemonmasters-game.com/en-US/announcements/Event_5090_1W_3