Just get a bazooka with 20 or more rockets. And an M4. So just start blowing cars with bazooka, the cops will arrive, blow up some more, blow up, blow up, blow up, kill, kill, kill. When the chopper arrives just start shooting at it with M4, it'll fill your chaos meter pretty fats. Or take it down with a bazooka.
This mission isn't as hard as it seems. Btw army will arrive when you reach the limit, keep that as a reminder how close you are to reach the goal.