Level 1-2 is where I got this done. Near the beginning of the level you will come to a bridge with a gunner at the front of it. While he's reloading, climb up the ladder and pick up a brick. Throw this brick to the back of the enemy below you. While he's distracted climb down the ladder and take him out. After doing this, go back to the gunner and let him kill you. You will restart on top of the bridge right near the pile of bricks. Rinse and repeat.