The Arena is a special type of challenge with an additional twist.

In the Arena, as you combo your attacks and stay aggressive the Crowd Gauge will rise. 

Once the Crowd Gauge is full, a new passive will unlock, increasing your combat effectiveness. Each passive is different depending on the challenge.

Maintaining a high Crowd Gauge will reward you with an increased effect to the passive bonus,  but letting your Crowd Gauge drop can cause you to lose your passive bonus entirely. Play aggressive to fill your Crowd Gauge and maintain your bonus!