This is towards the end of the Kreuzberg Headquarters mission which is the eighth of the campaign.
The main things to remember are to take your time, stick in cover and watch for the glint of the enemies scope to get an idea of where they are. There are 8 snipers in total but I only seemed to be getting shot at by a maximum of two at any one time.
I would also suggest using RB to go into focus mode as much as you can to go towards the Iron lung achievement.