Hard Headed: Achievement STEAM of Wolfenstein 2: The New Colossus

Helmets are dropped by dead enemies. You can use these to gain armor to protect yourself from damage.
Just kill lots and lots of enemies and pick up the armor that they drop. After the story you can still farm helmets by traveling to different districts on the map and killing the enemies there. You do this by interacting with the command console in the hub area (available after the story). There aren't enough helmets in the main story so you must keep farming them after the story.
Beware that starting a completely new game wipes your statistics. This has to be done in one playthrough / through post-story area select.