Part One: Preparation
The first thing you want to do is make sure that you either start undercover in the kitchen or make your way down there. If you want to kill Sergei without anyone noticing, obtaining the rat poison and master key to the palace is exceptionally helpful. Both of these items can be found in one of the side rooms, to the left of the kitchen. The rat poison in on the shelf and the master key is atop the stack of crates in the middle – pick these both up.
Now it’s time to make your way up to the second floor. My recommendation is to move your lovely hitman up the smaller staircase, just off the left-hand side of the runway. If you are having trouble locating this, the mini map will show you the location of the staircases dotting the palace. Enter this staircase cautiously, as there is a repairman at the top of the stairs who will spot you making your way up. Knock him out and hide his body in the cabinet at the bottom of the staircase. Switch to the repairman’s disguise and move to the second floor.
The next portion is more about personal preference, but I chose to switch to the auction staff uniform. Mainly because there are a few areas on the second floor, including the area where you will kill the target, that you are trespassing in if you are a repairman. With the auction staff uniform, you will only be frisked by these guards and not denied. Just, make sure you’re not carrying a firearm when you do let them give you a pat down.
Once you have the uniform, make your way to the large fireplace room just off to the left. If you are having trouble finding it, look at the runway from the second floor and follow the walkway left, until you reach the largest room. There will be an art room connected to it and a staff member on his phone. You can either leave him be or knock him out, that choice is yours. Just make sure to hide the body if you do decide to incapacitate him. Poison the wine glass on the far corner of the table and then make your way to the next set of rooms, north of your position.
Now the hit is all prepared, time to move to the next phase.