Win Fast Money in 10 Challenge Mode games playing with friends.
Please read "Veteran" achievement description first. While playing your Challenge Modes, have a second controller logged in as well and join the game with both (or more, if you really do have friends). Play through like normal using both controllers to answer. When you do the Fast Money Round, the MVP goes first and the other controller goes second.
How do I get my second controller logged in?
After choosing your theme and difficulty in challenge mode, you will see your avatar standing with the controller light above his head. Once your second controller is connected, press to join. You must choose a profile. Press on both controllers so that the lights above the avatars' heads are green.