Arena Mode takes a best-of-3 format where the first to win 2 rounds wins the match.

Each round takes the steps of preparation and combat.
You will be able to purchase Ability Books while waiting for the battle to start.
Once the battle ends, the purchased Ability Books will disappear.
You have to use different Ability Books for each round, meaning Ability Books previously used will not be available as you progress.
Tap on the ready button once you are ready to fight.
The battle will start immediately as soon as the opponent is also ready.

Try using the Status Device during the battle.
It will apply a Debuff Effect to the opponent.

If no one is yet to win near the end of the battle, the last player survives in the Veil will be the winner of the battle.