While hidden near an alarm button, shoot a bullet at a soldier to alert him. Then, aim at the alarm button. One by one, all the soldiers in the surrounding area will approach and try to press the button. Shoot and kill them before they activate the alarm.
Use the binoculars and adjust the pointer so that the enemy is in the center of the view. Close the binoculars and press [Left Ctrl] to hit the enemy once and kill them.
If an alarm goes off, find a location where you can hide but still have all the enemies visible. Shoot them one by one using a silent gun.
Make the best use of the thermal camera. You can surprise and kill enemies through the wall.