In the Rocket Facility level, you must defuse the bomb before anyone knows you are there. If they spot you, they will pull an alarm, the bomb will explode, and you will fail the mission. Try using the UMP sub-machine gun because it is silenced. You also have a motion tracker. If there is more than one enemy in a room, do not go in until they are standing next to each other so that you can get a fast kill.