For those who have not completed games in the Xenoblade Chronicles series, this page contains spoilers regarding the plot. Discretion is advised.
When the party first enters the Arena, Dickson seals the exit and sets the Obelis Obart on them. After defeating it, the floor drops down a level into the Battle Arena on 2F, and the Demon Pavlovsk enters from the Behemoth Nest which the party must defeat.
Upon leaving the Battle Arena, the story quest Chase Dickson is activated, and the party must make their way back to the Arena and fight the Master Obart. Defeating this third enemy causes a ramp to emerge from a wall, allowing the party to reach the Empty Throne and a transporter to the Great Canyon.