Those are fairly easy to get.
Every 'round', defined as everyone posting once, the 'arena' would be changed by 'a hand of God(or Liaison, or a Dreamer, or someone)'. Time would be stopped(I had an interesting exchange with a time manipulator during it). And restarted, like nothing happened, aside from the changing of the Arena.
The change would be to something random. The players could respond to things in the old 'arena', and then experience the change, and continue from there.
It'll be elimination style, of course.
It'll be noted that nothing in the new arena will be in the way to intercept an attack, to prevent people from 'lolchangetreeintheway', unless it can't be avoided, like someone making an arena where it's all walls and the like.