Skirmisher Achievement

Your secondary weapon is a sub-machine gun and is mainly used in situations where a rifle is not suitable. If you have unlocked the attachment, you can equip a suppressor so your weapon is virtually silent (they are unlocked by finding the SMG workbenches in each level). To unlock this achievement, you need to cumulatively kill 300 enemies with any combination of the secondary weapons listed above in Master of Secondaries (15G) throughout all game modes.
 
Refer to the ‘Mop Up’ section in the roadmap for a farming method to make this go a lot quicker.
 
To check your progress, press then navigate to Service Record > Medals > Weapons and Items and look in the first section to see your medal progress.
 
Note: For Master of Secondaries (15G), you are required to perform 150 kills with each of the six secondary weapons listed above, so this will unlock naturally while you are progressing towards that.