Make sure you have a large area with a big height, and start charging earthquakes. The next step is casting earthquakes on the middle of that area (always on the same spot), to create a circular arena. Note that, to make this process easier, you can place a fireplace on the centre of it, and cast the earthquakes directly on it. As the earquakes are used, the arena will start getting into shape, but it could take between 10 and 15 spells to create the perfect area. Then, do whatever you want inside of it - can be fun to create units wars or blast fights in multiplayer, or something like that!