This achievement is simple and easy to do. It can be earned on any difficulty, but to be safer and add extra challenge to it, I earned this achievement on 007 Classic difficulty.
Your recommended weapon of choice is the P99 Silencer. Make sure you stay hidden and take out your targets with head shots. You will see two guards standing next to each other, engaging in conversation. If that's the case, kill one with a head shot and immediately unload bullets onto the other quick before he's able to fire a shot.
Make your way to the area where you'll deal with two guards and a scientists via a breach. Kill them both and the achievement should unlock. Just make sure to stay undetected from beginning to end and it should be easy. It may take a few tries though, but with perseverance, eventually you will prevail and win!