A free, three-use arena mode unlocked by beating challenge difficulty and featuring an enemy deckmaster with its own cards.
Defeating Mushika at challenge difficulty unlocks Secret Challenge Mode. The arena labels it roughly medium-plus difficulty rather than a full higher tier.
Unlike ordinary simulations, the challenger carries and plays a deck. This lets Carl and Donut test snares, mystics, hand disruption, summoning locks, and deckmaster victory conditions.
The mode costs no gold but permits only three attempts. Its first match uses verdant hills and the condition that one strike against the enemy deckmaster wins, while any enemy totem or deckmaster strike against Donut loses.
A hidden arena menu opening three free deck-versus-deck attempts beneath a secret lock icon.
The mode gives the party its first realistic practice against hostile card tactics.