In the level where you first enter the road tunnel, go right to the truck. Blow up the barrel to kill all the little things. You can now either have an engineer fix the lights or not. Follow the left wall back to the door you came in and stop near it. There is a Walker locked up in some boxes. Throw a grenade to blow up the box and let the Walker out. Run back to the truck, or better yet, stand there and watch (if the lights are on). The guards not only get slaughtered, but really wear the Walker down. After he kills the guards, get him to walk around to you (not leaving the general area) then blow up the barrel. If you done correctly, you will kill the walker with ease. Otherwise, torch him.