i did this in single player easily. start up on the port of the passing and work your way to the generator picking up some adrenaline on your way. once at the generator, switch to expert and kill off your ai partners, then switch back to easy. Wait for the tank to arrive whilst staying in range of the original survivors laser sights. once you see the tank take your adrenaline and just run around the area near the generator allowing the original survivors to shoot him. once he's dead, the achievement pops. take care not to get any shots on him yourself.