The April 2020 issue of the Monthly Cococoro Comic, which was released on Saturday, March 14th, 2020 in Japan, comes with a serial code that allows players to receive special Dynamax Crystal (ダイクリスタル) for use in the Nintendo Switch games Pocket Monsters Sword and Shield! By using the Dynamax Crystal from this serial code, players can take on a special Max Raid Battle with Armorga's Gigantamax form. It has the Hidden Ability Mirror Armor which reflects the stat-lowering effects of moves and Abilities back to the user. You can challenge as many times as you want in order to win between March 14th, 2020 and May 14th, 2020 at 23:59 JST.

Source: https://www.pokemon.co.jp/info/2020/03/200313_gm02.html

April 2020 Corocoro Details:

Price: 600 yen (tax included)
Release date: March 14th, 2020
Format: A5
JAN: 4910038090408

Source: https://www.shogakukan.co.jp/magazines/0380904120