This one is really easy to get as long as you have a Gold subscription.
From the main menu, go to Multiplayer.
Choose Xbox Live, then Player Match.
Now choose Create Match and go into Match Options.
Set the round time to lowest possible, then auto-launch to 15 seconds. Once the first round starts up, simply do whatever you want for about an hour-and-half to two hours and come back. You should have the achievement now. It's the same for "Chaos Champion."