To gain a Special Rating you must kill every possible
hostile enemy on a particular mission with a miscellaneous / unique item. These items or weapons include such things as hammers, Baseball bats, nail guns, garden shears, kitchen knives, spades, screwdrivers, stilettos, etc. (Although in some cases some enemies can be killed by other means but most of the time you should kill them all with one of these specifically).
The civilians and secondary targets on these missions can either be left alone, sedated, or killed with the same unique item as you killed the hostile enemies with, however killing even one civilian with any other weapon has a high possibility of negating the achievement. In my opinion you should kill
every single human being (and even some animals if you can) with the same unique weapon, just to be sure.
One of the easiest ways to accomplish this is on the mission "A New Life" and on recruit difficulty (this is also much easier when you have the kevlar and painkillers equipped). Get one of the unique items on this map, either the garden shears, which can be seen in use by the woman cutting her privet on the opposite side of the road to the witness' house, run past her and to the back of her house, when she follows you try to grab the shears and kill her as fast as possible. You can also use the kitchen knife found in the kitchen of the witness' house or the baseball bat found in his garage.
Now the shears and bat are the most deadly but they cannot be concealed, the kitchen knife can be hidden and can be smuggled into the house via the caterers box. With the unique item in hand proceed to kill everyone on the map, if you are using the kitchen knife begin silently, kill the two garbage men around the corner, the clown, the jogger, the caterer and then move in and kill the easiest bodyguards one by one. If you get compromised, don't panic and reload, just try to get into a protected area and stab the enemies as fast as you can and use your painkillers at the best time. The methods with the bat and shears are basically from the compromised part, you will instantly be seen carrying those weapons and immediately fired upon, you must get into a good location with cover and kill enemies with stabbing or swinging motions. Behind the garden shears woman's house is a good place. Kill all the bodyguards and their back-up then move into the house and kill the target and anyone left.
Once you are satisfied that everyone is dead, make a quick sweep of the map and then finish the mission. If you used the Garden Shears you will unlock the 'Insane Florist' special rating, if you used the Kitchen Knife you will unlock the 'Sushi Chef' special rating and if you used the Baseball Bat you will unlock the 'Slugger' special and the achievement for any of them.
I've posted this video courtesy of 'GameVideos47' to visually illustrate the 'Insane Florist' method:
*Note that killing every single civilian and the guard dog may not be necessary.
This is a video, courtesy of 'Tomtokamoto' showing an alternative method on the same mission, he uses mainly headshots to gain the 'Russian Hare' special rating (you may want to mute this):
*Sorry about the music
Thanks and don't forget to upvote, please post any questions, improvements, or any new ratings you find.