On this page of the Wolfenstein: Youngblood trophy guide you will find the description of the “The Right Tool for the Job” achievement. This is a bronze trophy awarded for killing 75 enemies with the weapon that is most effective against their shield type.
This achievement only applies to enemies that use armor and shields, i.e. those that have white overhead markers in addition to the red health bar.
It is very important to read what type of shield a given opponent has. In the example from the attached picture, there are horizontal rectangles above the opponent’s head called Kugelsoldat.
When you bring up the weapon selection menu, look for those that have the same marks next to the ammo information. In the example above, one of the “suitable” weapons is the Sturmgewehr.
After encountering armored enemies, it’s best to look at the small graphic above their heads, retreat to safety for a short while, and draw a weapon that can weaken their armored armor. Try to use only suitable weapons after you have started to fight.