Arenas are special instances which task players with surviving while fighting waves of enemies. Once an Arena starts, players have a time limit to survive while also increasing their score by defeating enemies. Killing an enemy will grant a temporary score multiplier, which can be stacked up to ten times. Later stages have more difficult enemies which will award more points. The more points that you obtain, the higher your rank and rewards!
In order to gain access to the Arenas, you must first complete the side mission chain that gives you access to them. You can locate this mission by checking your map and finding the Arena icon. You must complete each Arena in order to access all five.