To get the FBI to your party, you will need to kill 2 regular cops. To do this you can set up traps for them to run into or have good timing with multi use traps like the horse. I was able to get this achievement on the Bike Party level when it had a horse loaded at the entrance of the building. I was able to kill people inside the building so that the cops would be forced to walk right past the horse and I could kill them. Once the 2 regular cops are dead, the FBI will show up. If you can, stay away from them but if they do notice you, use trap doors and run. They will give up chasing you just like regular cops would. After they are all gone, just finish the level normally and the achievement will pop.
This can also be easily done on the Casino level since there are two traps lined up in a row for you to use. On the left of the level there is a gumball machine and a oven that you can rig to kill the cops as they walk past them. Just get caught killing someone, rig the first of the two traps and make sure no one runs into them other than the cops. Then repeat with the other trap. Also, stay up in the top left corner by the escape and use it if needed to avoid getting busted.