heres some additional info that u might find useful:
Its not just the mage areana, When u chose to attack something, u are able to pass through walls to get 2 it. The mage was actually in an area where u cant get to, i was just able to phase through the barrier.
I discovered this in the kalphite tunnels. I went to attack a gaurdian, and i got stuck in a wall. I then chose to attack a soldier on the other side of the wall ,and i walked through it