So... when you do the "Lady Boyle's Last Party" Mission, and you do not have shadow kill, and your given target is Lydia Boyle (the one in red), you will be led to music room. There, she will ask you to play a song on the piano. However, as the music room is open to guests and servants, once you kill her / knock her out, you will attract unwanted attention... how do you eliminate / neutralise her without attracting unwanted attention? (Let's say I have shadow kill tier 1. Even then, she won't turn to ash because she already noticed you. Am I right?)
Wait for the opportune moment... a long time...
She actually goes to the toilet sometimes. Kill her there and leave. If you want to keep her alive, it's a bit harder...
Firstly, you will need to reduce the number of unnamed guests in the room to the right to the opening hall. Knock them out and dump them in the room with the Wall of Light controls.
There is one guy there you can talk to. Go bother him until he gets the guard from across the room. Lure this guard into a quiet corner and remove him as a problem. Now you can get rid of the Overseer that was previously watched by said guard.
Proceed to remove as many guests as needed from this area (you can poison-dart everyone there quickly), grab the body and move upstairs quickly. From here, proceed to the stairwell at the other side of the mansion, down through the kitchen and deliver her.
If you go to this site, you can read all about the whole mission, including how to take Lady Boyle non-lethally.
"If Corvo talks to Lord Brisby, the partygoer wearing the rat mask in the smoking room, he will reveal his undying love for Lady Boyle. He will say that if Corvo can deliver Lady Boyle to him, Brisby will make sure that she is never seen again. As such, the mission can be completed by rendering Lady Boyle unconscious and delivering her to Lord Brisby in the cellar."
After you've talked to Brisby, and you've identified Lady Boyle (she can be any of the three), you can talk to her and get some dialogues. If you choose the right dialogues, she'll start walking down to the cellar with you. And when you've gone down the stairs and past the kitchen, you can take her out without anyone seeing it.
One option is if you have the Bend Time II. Trigger Bend Time, knock Lydia out, then teleport away. You only have about 8 seconds of safe time, but I found it easy to pull off in the main hall (the one with the Light Wall). It might be helpful to Bend Time a second time to give you plenty of time to escape.
Another option is if you have Possession II. Possess Lydia and move her into a room, close the door, then remove the possession. While she is disoriented for a second, take her out.
Note that Lydia will path fairly close to the room leading to the cellar, you should be able to get directly to the boat without much trouble using either of these methods.
Another good place is the room with the Light Wall. You can either Bend Time and teleport up with Blink II, or Possession will last long enough to move her into a room upstairs.
Please note that with either of these methods, it may be necessary to take out any nearby Overseers. Their music boxes will render your magic useless if they catch you and stop playing.