My idea is that the monsters in the arena must stay in the square they were summoned from. When training my newb crits it is very time consuming to have to clear a flooded area...then someone runs in and 3 more monsters enter...
Bah, I don't get why people even use arenas when it's going to take you twice as long to level. If it weren't for BoD & Pile, I would probably still be in them.. they inspired me to go die a little.. since it is part of the game.