To unlock the Unseen Assassin trophy in HITMAN 2 you simply need to take down any Target (i.e. an individual you have been instructed to kill) without being seen by anyone.
You can do this in any level and also achieve this in community levels and Elusive Targets, and in order to complete this list you are going to have to get comfortable with a stealthy approach anyway -- so you will likely unlock this naturally.
However, you can get it out of the way easily with the Hawke's Bay mission "Nightcall", particularly because you have free roam of the location to scope out opportunities and pick up items for as long as you like -- as long as you don't put the security dongle into the computer, which triggers the arrival of the Target and their guards.
There are a number of ways you can do this but bear in mind that you need to remain unseen and the murder undiscovered. You can try smothering the target with a pillow once they go to bed or strangle them while they are cleaning their teeth and their partner is in the shower, but bear in mind you need to be quick about dragging the body to the wardrobe to remain unseen.
You can also use a couple of tactics to get the target into the Panic Room by the computer and then poison them via the ventilation valve on the roof of the building. You'll find a poison vial in the cabinet in the bathroom. You can cause an alert without being seen if you are careful about timing and positioning just by throwing a chaff grenade into the house, but you need to make sure the target is the first person to go into the Panic Room.
Another less risky but more time-consuming method is to grab some chloroform from the garage and spike the target's tea before they drink it. When the target starts reacting she will be hustled into the panic room, and if you have already poisoned the air flow in there she will die.
However you choose to get the target and get out, as long as you get away unseen you should get this trophy. Just bear in mind that you still need to distract the guards standing around your boat to escape, which you can achieve in a few ways such as shooting the canister in the back of the parked truck.