For this hidden trophy you have to successfully administer a dose of Anesthetic on either: a guard, civilian or a target then the trophy will pop immediately afterwards. You get the Anesthetic in every mission, there are one or two levels where you do not start off with it but it can usually be found somewhere within the mission.
To use the Anesthetic equip it from your inventory and find an isolated victim. Preferably finding someone who is stood still and press to sneak up to them. Once behind them press and hold until 47 lets go. Note: When 47 gets out the Anesthetic, the use of it will be disabled for about 2-3 seconds as he has to take some time out to prepare it so factor that in. It would be wise to equip it a couple of feet back, leave it too late and you will be left red faced as you may bump into your victim which will result in being spotted!
The earliest you can get this trophy is in the Mission 1: Anathema as there is a guard who goes to the toilet right at the start near the door, however, the most convenient place is in the Mission 3: Kirov Park Meeting. When you exit the sewer you can subdue the Limo driver for when he goes for a toilet break. Nobody is around and you will need his disguise if you wish to blow up your targets with Bombs so it fits in conveniently.