The mission you want for this one is 'Numbers' and I suggest doing it on recruit as the game is already hard enough.
Some tips that may help:
-Small short bursts will help you be more accurate.
-More dual weapons can be found as you go through the level.
-You may pick up non double weapons, as long as you don't use them. So if you accidentally pick up a non double weapon you don't have to start over as long as you don't use it.
-You can run past most of the enemies that are far away (in windows, on roofs ect)