First of all, dont use the "run" button, it's useless because you can just simply spam your Mouse Button like hell and youll be as fast as sanic (I'm not joking, it realy works).

The second is pick the 2nd guy with the silencer, seriously, the assault rifle guy is pointless because every damn terrorist will be aware of your presence and you die.

The last Tipp is: Aim always for the legs/ shadows of the terrorists since the hitdetection only works for them.

Oh and just watch out that you stay out of sight. I will later upload a video and link it to this guide to show, how it works