Complete all instances of Mayhem.
Instances: 6

In this activity you have to rack up a certain amount of points by causing as much damage as possible. You will usually be given an RPG with infinite ammo, if not then infinite frag grenades, so spam that sucker to your hearts content. Fences, trash cans, and other small items are your best friend - they don't give much money individually, but they increase your multiplier (and thus your money) insanely fast.