Call of Duty: Black Ops III

You should get these easily about 10 minutes into the campaign. Lookout for the section in the video below (props to it's maker)(it is truly the first section of any real action). If you don't get it, restart the checkpoint and approach this tunnel again.

This achievement requires you to kill any enemy through any type of obsturction. It is most likely you will get this accidentaly, but if not here's an extrmely detailed walk through on the best way to do this.

First of all, any bullet weapon (basically everything besides launchers/ knife) will be able to penetrate a wall at varrying degrees. The most effective weapons to use are light machine guns, as they have the strongest wall penetraton factor, compared to every other type of gun. So from the get go make sure you create a class and equip a light machiner gun of your choosing (I used the dingo) as well as the attachments full metal jacket and the thermal sight. FMJ greatly increases your wall pentration, so that combined with the LMG means you can do alot of damage through walls. The thermal sight will make it easier to identify targets.

Although this can be done on basically any mission, I found it easiest on the second mission "new world" load this up on any difficulty (preferably recruit) and watch or skip the into cutscenes.

As soon as you get control of your character you will be in a firefight. If you look directly forward and up you will see a number of enemies behind some barricades, aim dow sights to identify a target with your thermal sight and move your aim down a tad so you are shooting them through the wall. Keep shooting this area until the enemy downs (shouldn't be more then 2 or 3 shots) and BAM! achievement pop!