HITMAN

Here is my strategy / video guide to solve this mission. If you follow the strategy / video you will obtain "automatically" the Silent Assassin, Suit Only Challenge and eight other challenges (see list below)

My strategy is that I kill Dalia as first and then I kill Viktor and leave the mission. Ok here are the basic steps:

Dalia's Kill

1) reach Dalia's bathroom in the third floor via pipe outside the building
2) Subdue the guard in the room next door via radio distract
3) wait that Dalia come back
4) distract Dalia's Bodyguard with a overflow sink
5) kill Dalia with Fiber Wire and hide the body
6) leave the area via bathroom and go in the basic / first floor

Viktor's Kill

1) as we come down from Dalia, I go to the room with the Diorama and hide behind
2) wait that Viktor is close to the Diorama and insert a coin in the Diorama
3) when Viktor leave the Diorama, subdue the bodyguard near the door and hide the body
4) wait that Viktor cross the same door
5) kill Viktor with Fiber Wire and hide the body one floor below (basement)
6) leave the mission through the Catacombs exit (basement)

All Challenges in this vid:

The Mastermind
The Moneyman
Piano Man
Silent Assassin
Silent Assassin, Suit Only
Suit Only
The Personal Touch
Suit Only
No Evidence
On to the Catacombs

There are many ways to complete "The Showstopper" in Paris.

The mission can be completed in about 8 minutes with a Silent Assassin, Suit Only rating pretty easily. This is the recommended method as it gives the most XP towards Paris mastery which is necessary for other achievements.

This strategy can be seen here:

I have made two videos, the first is a silent assassin run and is more recent.

Some of the challenges don't show during these videos because I have already reached max rank and completed most of them. So with that said, you might find you complete a few additional challenges I haven't noted.

Silent Assassin
Start with this video as there is no mastery required and you can get Silent Assassin straight off.

Targets:
Dalia Margolis
Viktor Novikov

Challenges and Opportunities completed during this playthrough:

To Your Very Good Health | Poison Dalia Margolis 5000XP
Knockout Punch | Assassinate Viktor Novikov with a poisoned cocktail 5000XP
Silent Assassin | Only targets killed 5000XP
No Evidence | 1000XP



Various Challenges
This was the first video I made and contains the same method of eliminating Dalia but still you'll make some reasonable XP gains during this playthrough by completing everything else.

Challenges and Opportunities completed during this playthrough:

To Your Very Good Health | Poison Dalia Margolis.
15 Seconds Of Fame | Take to the catwalk as Helmut Kruger.
A Rare Scoop | Find the camera lens. Give it to the blogger.
Photobomb | Arrange the interview with Viktor Novikov. Conceal an explosive in the camera. Assassinate Viktor Novinov.


I hope this helps but any suggestions to improve the solutions are more than welcome. If you are going to down vote, please let me know why so I can improve the solution.

Here's the playlist for the rest of my Hitman videos:
https://www.youtube.com/playlist?list=PL1nCuO5R6HvFWO4RGIYOa...

I wanted to add that you need to make a save right before you finish this or any of the other Episodes. That way you can do the partial run for Opportunities or Challenges and just load up this finished save and hit the End Mission button.
This can also help pop some of the achievements that require you to finish to mission to receive. For ne this was