In the final part of the Der Flakturm mission, when you have to escape through the sewer, you will meet about six of the hard to kill elite machine gun soldiers. It is almost impossible to defeat them all. It is much easier to run past them all. Make sure you have at least three health bars. Once you climb the ladder the game is all but over.

In the sewers, if you are skilled enough with the MP40 you can take down all the elite machine gunners by hiding behind something and waiting for them to walk by. Shoot them in the head. With the fully upgraded MP40, it takes nineteen headshots to kill them.