This will come by naturally playing through the game. The Darkling guns are acquired in Chapter 2. It is a form of The Darkness. It will be the green-eyed Darkness form. You will be able to use two special guns with this. One gun is a mini-cannon and will usually kill in one hit, but takes time to recover from the shot. The second gun is a mini-machine gun. This gun is not as powerful as the other gun, but it is extremely fast. You can usually shoot down a group of enemies with both of the guns.
The first place that this achievement will become available is the first time in Hell. Just shoot the enemies from a distance and you will get receive the achievement soon. It is recommended to do this on the train that you will attack and take over in Chapter 2.