Select practice mode choose Kilgore, opponent does not matter, I used Thunder. Once in practice mode hit the menu button, press RB twice to get to the practice options menu, set your instinct meter to Infinite, press B to get back to your practice battle. Once here activate your instinct and then do any of your chaingun attacks until the achievement pops, I used half circle forward with the right bumper and kept hitting it over and over until i got it.