In "The Showstopper" mission, get a disguise (Security Guard works fine). Read the stage plan (found behind the fashion stage). Go to the room where the models are being prepared for the stage. Wait for Sebastian Sato (the fashion designer) to arrive. Then, insert a coin into the music box behind him. The music will make him walk away. After he drinks from the wine glass, he will walk behind a wall where you can take him out without being seen (save the game when he starts walking in case you make a mistake). Then, go to the top floor, get a crowbar, and climb the light rig above the stage. When Viktor Novikov comes on the stage, use the crowbar to drop the lights on him (he takes a few minutes to arrive) to get "The Showstopper" achievement.