For those of you who simply cannot beat this, there is a less than legitimate way to obtain the achievement. Start the dlc on whatever settings you like, pick your weapons and go into whatever "scenarios" are available/ you have to beat. As soon as you enter the scenario and the timer starts counting, open your inventory (default LB) and try to equip your gas mask repeatedly (HOLD LB and rapidly tap X) you should be told you did great or amazing or whatever and the scenario will end. Repeat for all remaining scenarios. You can complete the entire dlc in 5 minutes.