Been playing Deus Ex on the hardest difficulty setting. 'Realistic'. To sum it up, if you get discovered you die. Unless you do fancy tricks with gas grenades or ballistic armor or something. There was one part where I was on top of a building and had to work my way down through three levels of soldiers, all in the open and all of them better armed and better shots than me. I just thought 'fuck it', jumped off the side of the building, then repaired my legs with med kits and strolled off without killing anyone. It's the way I'd do it if I was really an agent.