No "Rules" Really, as far as I know just only attack people standing in the arena.
I (Rhynn) Went there for the first time yesterday.. Usually two people stand at opposite corners, and buff if need be, and there is someone in the arena off to the side..
That person counts down from three to one and says ~Enjoy (Well, That's how It was done)
and the battle commences...
Really, Don't go into the arena for hopes of victory. its just a place to test out new spells and stratagies in a safe environment
~Rhynn really has to stop quoting people.