it takes practice, over and over again, new strategies. it take time. in the room where u said u killed the guy through the curtain u can either crouch and take him hostage or u can throw a can to the left , where there's a dark spot, when he walks over, go to the computer get the stick and go back out, he'll be too busy looking at the can to notice u.

btw in case u didnt know this yet, to roll, u have to run then hold crouch button.