Arena

The Arena is unlocked after you reach a certain level in the game, it's pretty early, about level 15 I think. It is the games PvP element where your team is pitched in battle against another human opponent's team albeit your opponent's team will be auto controlled by the game itself.

Once you have selected your opponent, you then select your own team, and you then play a one round battle with a 5 minute time limit. If you have already unlocked the arena then you would have already also unlocked the speed up mode and the auto mode, which allows the game to control your characters and the battle would be over relatively quickly.

If you win the battle you take your opponent's position on the leaderboard and receive the respective reward for that position when rewards are distributed.