31 wow how the hell? You must be awesome. I never played the hard or insane maps on solo just went in to them with 2 player. In fact I only ever got to wave 24 as my highest on two player, but then I was playing with a random rather than a friend.
In Co-op we have reached wave 57 on Seatown, 39 on Dome and 31 on Arkaden. We are in Afghanistan so our internet is too slow to play online. I was wondering how we stack up on the world stage???? I haven't played much solo.