Shoot the tankers while they are on the helipad. Wait for Warhawk to hover over the helipad and explode the tankers. Warhawk's shields will take damage. After doing that about four times, Warhawk's shields will be destroyed and the Tankers will stop appearing. Remember to restore your health often on the level, and hide when you need to. Also try using the "Satellite" weapon for major damage.