During the final part of the "Der Flakturm" mission, when you must escape through the sewer, you will meet about six of the elite machine gun soldiers. It is very hard to defeat them all. It is much easier to run past them all. Make sure you have at least three health bars.
In the sewers, if you are skilled enough with the MP40, you can take out all the elite machine gunners by hiding behind something and waiting for them to walk by. Shoot their heads. With the fully upgraded MP40, it takes nineteen headshots to kill them.