So this is a very basic achievement which has you get 5 kills with the assault rifle without reloading. You get the assault rifle very early on in the tutorial mission. Once you have it, simply line up your shot for 5 headshots and you can get this with the next 5 enemies you see.
If this does prove to be difficult, you can wait until you can upgrade your assault rifle (using 250 feltrite and 1 weapon core mod) to increase the capacity to 52 instead of 35.
Skip to 0:38 for Efficiency