When you are in Saulnier Cryogenics, on the level "Paris 2", you have to deactivate a bomb, there are two guards near the boiler room.
If you wait like 30 seconds, they will leave and then you don't have to neutralize them.

Equip your thermal vision goggles and the ones with a higher shade of red (heat) on their body are terrorists due to the side effects of the nd133.

When in the dark and can't exactly get the guard where you want him get close but still in the dark and press L2 to whistle. The guard replies sometimes like "What the" or "What the Hell". He walks to the place in which he heard the whistle and turns and stops then starts walking. Stay crouched and go up behind him fast and hit R1 before he gets out of the shadows and it will hit him in the back of the head making him unconscious so that you can carry his body into the shadows to dump it.

During any level they will have lights in it. Shoot out any lights that you can see and have an accurate aim to shoot at. Don't even try to shoot out the lights with the cage around them. Oh, and before I forget, don't shoot when a guard is around because they'll think their under attack.

Find a dark area to hide and then press R2 to whisle which will make a guard approach the sound.

Then jump the guard when he stops.

To detect the mines more easily in the Camp mission turn on the thermal vision.

To move quietly and remain undetected longer crouch down and walk slower which will make your steps softer.