Splatster Chief Achievement
Complete all levels of Septic Avenger
In Septic Avenger, you work the pumps on a rogue septic truck, with the objective of spraying waste on everything you can. You need to spray virtually everything in your path to get the required amount of money, especially in the later levels. The good news is that your truck has nearly infinite health and there's no time limit. The only thing that will eventually stop you is when the truck finishes its route, but you can stop the truck by holding LT.
There are 2 Septic Avenger activities with 6 levels apiece. I recommend doing this in co-op so you can work towards the "Confidence Men" achievement as well as "Crime Lord".