I found it easier to get it on the Streets,after the RETOMOTO Tower,when you head to the Bus Station,play as Lynch (Co-op).
Lynch will hallucinates and you can shoot all the civils that are crouching there,and there are no mad guys interrupting you.
You got about 1-2 Minutes till the hallucinates ends,and have to kill about 10-15 People.
EDIT : You can do this in LOCAL CO-OP with 2 Controllers.
Just make sure your Profile is Lynch.