As robrargh said, Careful use of Napalm is the key for the hard badge. Also learning to avoid that 5 cars + airstrike is also necessary. I would also recommend to fully upgrade everything. One big advice I would like to give is take Rocket Launcher with the leader, also try to have at least 5 AA Specialist on the team. I had 6 and I got to 4000. The more missile, the better.
Don't listen to other people, Homing Missile is super useful. When I was at the last gate, all my allies died.. I was all by myself and only 2 bars of life remaining, I had to dodge every bullet paying attention only on the lower part of the screen. In this situation, homing missiles helped massively in completing the game.
A little suggestion: Since the art/graphics/scenes are all somewhat of the dark tone, it would be soothing to the eye if the dialog-boxes are of dark color with light color text. The main menu was dark, so I decided to play in full screen cinematic mode to enjoy the beautiful art. Then suddenly my eyes get blasted with white light from the dialog-box.
 "Shift"){kind=icon}
 "SHIFT 3"){kind=icon}
 "Symbiosis"){kind=icon}
 "Sugar, sugar 2"){kind=icon}
 "40xEscape"){kind=icon}
 "Blocky XMAS"){kind=icon}
 "Battalion Commander"){kind=icon}
 "Splitter Pals"){kind=icon}
 "Balloon in a Wasteland"){kind=icon}
 "Rhomb"){kind=icon}
 "BackDoor- Door 2"){kind=icon}
 "Momma's Diner"){kind=icon}
 "Get A Little Gold"){kind=icon}
 "The Man Who Can't Jump"){kind=icon}
 "Trader of Stories - Chapter 1"){kind=icon}
 "a Grain of Truth"){kind=icon}
 "Hordes and Lords"){kind=icon}
No Robot Chicken references, but now I'm kind of curious what you're referring to.