Kill 3 enemies using sniper scope while wearing the Gas mask and using the Night Vision goggles.

This achievement can be unlocked during the Volga level. A sniper scope is a scope that has at least a 4x magnification so you must first find one of these. They may be found on guns dropped by defeated enemies, however you can find a scope in a safe house towards the south of the map. The Gas mask you will have from the start of the game so now you need to acquire the Night Vision goggles. You can find these at the end of the mission to commandeer a rail car. In the room where you can fight the giant catfish there is a door on the opposite side to where you entered. Go through here and straight ahead there is a shrine with the goggles on it.

Once you have all three, put on your gas mask by holding DPAD 'DOWN' , press DPAD 'LEFT' to turn night vision on and go and find a group of bandits. You will need to steal a passenger rail car during the mission where you can acquire the Night Vision goggles and there are around ten or so enemies here that can be killed while not having any effect on the "Duke" achievement. Be sure to aim through the scope when killing the enemies. See the description for "Gunsmith" for a location to find a scope for the Kalash assault rifle.