For this achievement you need to get 100 dual wield kills. Now you can dual wield almost every gun in the game. I would advice doing this with dual assault rifles as once you get use to the aiming this is good in battles and can kill them quick. Also in sneaking sections use two dual pistols. Very useful for guys that take two headshots to die. And dual shotguns for the heavy enemies. This will come quick and you will still probably use dual guns after this achievement