Do you remember at the beginning of the level when Shetland (if you chatted with him) tells you about his having snipers in place?
It's the red dots from Shetland's snipers that Lambert is reffering to.
You can end this mission several ways:
1. If you choose to go the run and gun method, kill a few then run like crazy for extraction and Shetland's snipers will finish the rest of them off.
2. To prevent ANY deaths for a zero kill game, before opening the final door, arm a flash-bang. As soon as the action starts after opening it (you'll first have your hands in the air, IIRC) immediately back up to your right so they don't turn you into swiss cheese, then toss the flash 'nade through the door. With your thermal goggles on you can run right through the middle of them without firing a shot and the snipers are also blinded - preventing them from killing anyone!
