So this is pretty simple and easy with a vehicle mount.
Take your vehicle mount into the sewers of Dalaran, more specifically into the Circle of Wills. Go to the corner of the opposite end of the arena masters. Right here:
Now make sure you are positioned like you are in the picture and then exit the vehicle, you will be pushed through the wall to your left and just turn your camera a bit and you will be able to see the underground arena:
Now you can just hop down behind you into a thin layer of water and then just drop into the arena and see the NPCs fighting:
note: there is NO exit from this area so you must hearthstone or portal out. I brought my friend on the mount and took him down there while he had no his hearth on CD, i hearthed out leaving him for a few mins and he was quite ticked thinking i wasnt coming back to get him!6):