As Samuel is dropping you off, he mentions that the security has been improved with the addition of Tall Boys, which are guards on stilts with explosive bolts as firepower. Go up the first set of stairs and you'll be on street level. If you cross the bridge right there, you'll find yourself face to face with two Tall Boys, as well as a normal guard. If you go to the left of the bridge, you'll find a Tall Boy and two guards.
Unless you can get above the Tall Boy, it is best to leave them alone, as it can be hard to fight them if they are after you. Start off by going past the bridge and into the alley near the large bridge. Get up high and Blink to above the Tall Boy, then press the button to Drop assassinate him. After you take him out, kill the other two guards.
THE PARTY
Go across the large bridge, being careful of the Tall Boys that patrol to the right, and get over the fence. Make sure as soon as you get over the fence, that you put your weapons away, as you will not want too much unwanted attention. You do not want to attract too much attention here, as there are A LOT of guards.
If you got the invitation from the Art Dealer's Apartment in House of Pleasure, then walk up to the gated window and give it to the guard to get inside. If you don't have that one, walk towards the guests standing ahead of you and the one lady's invitation will fly away. Go and pick it up, then give it to the guard to get inside.
(OPTIONAL) THE DUEL
When you get inside of the compound, head torwards the optional objective marker and talk to Lord Shaw. When you were leaving The Hound Pits before this mission, Pendleton gave you a sealed letter, which was actually a duel request with Lord Shaw. When you talk to him, he and two guards will start walking over to a designated spot.
When you get to the spot, choose your gun and then walk over to the designated spot for the duel. Keep your back turned and the countdown should begin shortly. When it reaches two, start turning around and aim well enough, then fire and kill Lord Shaw. If you happen to get shot, then you will die and have to load a save.
IDENTIFYING LADY BOYLE
Once your duel with Lord Shaw is over, put your weapon away, head inside the estate and join the party. Your first goal is to identify the real Lady Boyle. There are three of them, and the real one is chosen at random every time you play the mission. Of course, there is a way for you to find out who the real one is.
Find and talk to Lord Brisby (scarecrow mask), who knows what you're at the party for and he wants to help you out. Now, he is part of the nonlethal way of doing the mission, but he will help out in isolating Lady Boyle. After that conversation, find Miss White (moth mask) and talk to her, who mentions that she needs a drink. Fetch her one and she'll mention who is who.
Find the Lady Boyle and talk to her. To make it easier to kill her, steer the conversation until you get to a point where you mention someone is going to kill her. When you get an option for her to meet you in the cellar, take that choice and follow her. When she goes through the kitchen and out of view of anyone else, kill her.
There's a switch nearby, so hit it to open the door. Don't worry about Lady Boyle's body, as no one comes down here. Right around the corner from Brisby is a door that leads to the sewers. Use the valve to open the door and it's a free swim to Samuel and the end of the mission.
