The Monster Arena can be opened as a separate instance via the NPC Cruix (main_office 117, 159) in the Main office.
Here you can use monster summoning items like Dead Branch (604) and Bloody Branch (12103) .
This map contains also a Healer, Storage and a Tool Dealer.
Prince Croix can help you to get rid of unwanted monsters on the map. (party leader only option)
The NPC Wise protects the NPC are from the monsters.
It's impossible to fight on the bridge.