There are two septic avenger locations on the map that are really quite easy to do - each one has 6 levels in. One is at the very top right of the map and one is at the top middle of the central island - this website will help you locate them easily:
Map no longer available, sorry.
You will use the RT to spray the hose and LT to stop the truck should you wish to get a more accurate shot or spray more than one thing in the area. You will not need to worry about driving. Try not to rush it as a progress meter in the top left will tell you how much time you have left so make sure to hit as much as you can - the round will also end if you reach the specified amount of cash.
You will be shown on the map by large red markers which targets you need to be spraying and a type of health meter will be displayed when aiming at it to let you know that you are nearly done painting it brown. Spraying these will earn you big money as they tend to be things that would be quite expensive in real life.
You will be pursued by various members of the law so I recommend that you fill their vehicle up with some raw sewage for $1000 a piece but due to how often they attack it all adds up. Try to memorise what things earn what money when spraying them too.
Doing this in Co-Op will dramatically increase your chances of beating the levels but make sure you aren't too hasty.