Try using the luminous gun they have on the table when you start. This with the sub-machine gun on the table. I did and I was able to significantly reduce my time. Also, memorize where each type of villain is and be ready to shoot before appearing. When you have the gun, place yourself in one place and take all, then move on to the next place. With the sub-machine gun, you can usually shoot the hip and get your target by advancing. I have not had less than 30 seconds, but you only need less than 35 to get the 3 stars, and I got that.
Call Of Duty Modren Warfare 2 Special Ops The Pit walkthrough and How to Get three Star Like A Boss. See the video and try yourself. Definelty you will get three star and may be you can complete this in minimum 30 second or less.