I found the Mayhem missions really easy in the end. As long as you have infinite ammo on the rocket launcher (which you will do most of the time), just stand there blowing up everything that comes by to get your police rating up. When it hits 4 stars take out the chopper right away, which will get you to 5 stars, which brings the FBI vehicles, which take 2 RPG shots to kill, but give mucho money when you blow them up. I did level 8 each time with over half the time remaining.