This achievement is pretty simple and can be tried many times until you actually complete it. Obviously this is done in Episode 3 - The Mole, The Mob And The Meatball.
First you need to go to Ted E. Bear's Mafia-Free Playland and Casino (just drive the Desoto). Once you enter you will be given some tokens. Use the tokens with the WHACK DA RATZ machine.
You will then have to shoot the orange rats while avoiding the yellow rats. You need to get a score of at least 20 to get the achievement. Although the description says "perfect score" I hit 1 yellow and still got the achievement with a score of 21.
If at first you don't succeed just keep trying till you unlock it. It shouldn't take it too long. I got it on my 2nd attempt. Good luck.