For this acheivement, you have to kill 20 Germans on a mounted gun.
Whilst you come across a few mounted guns in the initial level, these are no ideal for mopping up kills.
Later in the level, you will come to a courtyard type area on multiple levels, with 2-3 mounted guns and Germans firing above and below on your men. Just mount one using X and blam away. I found it easiest to aim for their light blue backpacks. If you don't get it initially, just reload checkpoint as it keeps you stats over.