Go straight to the wall by Target A (search and destroy mode) then find the blood stain, sprint into and out of that corner repeatedly until you fall through then duck down crawl through the gap and your under the level from here you can kill people but they cant get you - be warned you cant throw grenades through to the level. Also from there if you want to get out of the whole level instead of under it just walk backwards towards the wall on your right you will pass through but don't go too far or you die, from here you can see through nearly all walls and kill people without being spotted.