Tell the arena master you want to watch a battle, and bet if you want to. When you are in the box, try to jump out. Sometimes you will get stuck between the ledge and the ceiling, and I have gotten 50+ damage for falling for no reason (I didn't even fall). But if you try things like running around, facing backward, gaurding with your shield, ect, I have managed to fall out and onto the arena. Still, I don't know a precise way other than try to worm myself out.