Using digital cards and special rules, up to four players can compete together to take down a Dynamax or Gigantamax Boss card. Each player uses two of their own real Pokemon cards to fight this boss, which has an HP in the thousands and can attack multiple times per turn.
To play, you can load a Digital Battle Assistant or print out a few materials from their website: one boss Pokemon card, a deck of 20 Boss Attack cards, a deck of five Cheer cards, four Knock Out counters, a playmat, damage counters, and a coin. Each player also needs two of their own Pokemon cards.
The full gameplay instructions can be found here.