Guys, just in case you might be struggling with Flawless achievement (which you will have to earn Grade S on every level). I found an easy way to complete it. All you have to do is set the difficulty to "Tourist" and play on. Guards won't react even if they see you, meaning you can confront any guards, stand in front of them and so on. None will chase you. Just remember - Don't knock them out.
I found this method after I've done playing story mode and didn't really want to start again so I tried changing settings and, GOTCHA, found a way. There was no guide for this achievements so I thought it would be useful to all of you.