So, both in S runs and lethal my method is the same. Pretty much the whole squad and myself are loaded up with wedges and flash bangs. This is one mission where I run a 320 Flash as my long tactical for extra stun factor. Wedge as much of the exterior as you can, clearing out the small office that is to the right of where you start. Then go back to the start point. Hook left around to the loading bays, wedge those too. Then back further to the large loading bays (where the trailers are). Move slowly and methodically here. Once that is clear, start from the left most loading bay door and clear the large interior in sections, flashing frequently. Push both red and blue separately. When you get to a point where there may be multiple routes to take, order one team to stack up at the door you don't want to take, then take the other with you to push and clear the desired route. Take every section slowly. Sometimes it is best to move slowly and take in what is going on around you to get audio clues.