In co-op mode, one person should be a Medic, the other Special Ops. Kill all enemies in an area with an intact (un-exploded) car. Blow up that car, then have the Spec-Op walk into the fire. Have the Medic constantly heal the Spec-Op (with the secondary fire from the XR-42), while the Spec-Op should toss ammo back at the Medic. *** Obviously, this won't work with a Soldier unit.