Overview
This is quite a hard achievement to get, as it requires you to complete a series of missions in which you have to defend against a number of waves of enemies. However there is a glitch that enables you to get the achievement in about a minute without even firing a bullet.
Here’s how to do it
Load up the mission and enter the virtual reality combat simulator. Select any random weapons as you won’t need them anyway and then proceed to start the first mission. As soon as the mission starts hold Tab for the weapon inventory and spam G for the gasmask. After a few seconds the guy will say that the mission is complete and you will return to the mission hub area. Repeat the process for the remaining missions which will take a minute or so.