Go directly to the wall by Target A in Search And Destroy mode and locatethe blood stain. Sprint in and out of that corner repeatedly until you fall through. Duck down and crawl through the gap and you will now be under the level. From here you can kill people but they cannot get you. Note: You cannot toss grenades through to the level. If you want to get out of the whole level instead of just go under it, walk backwards towards the wall on your right. You will pass through. Do not go too far or you die. From here you can see through nearly all walls and kill people without being seen.