Silent Assassin is the highest rating you can receive at the end of a mission and you can earn it by being the perfect assassin. Things to consider when trying to get a silent assassin rating are how many people were killed, as only your targets should be killed. Also, how stealthily you carried yourself throughout the mission, for instance, not being caught in a disguise or a restricted area. Killing civilians and police is completely unacceptable as is killing guards and patrolling enemies. You should try to avoid all contact with people during the missions and only kill your target. You should only kill in a stealthy fashion too, shooting someone, even with a silenced weapon, will void the silent assassin rating. You should try to make kills look like an accident by exploring the possibilities of how you can kill your targets.
You will be able to see what your score is at the end of your mission. You can have at worst, 1 'alert' OR a 'no target killed' in your performance statistics and you can still attain a silent assassin rating.
These ''5 Silent Assassin ratings'' achievements are stackable, so getting 5 Silent Assassin ratings on professional will unlock the lower difficulty achievements, too.
You can obtain this achievement, by replaying the first mission 5 times and getting the silent assassin rating each time, if you want to get it very easily.