Use the front door to enter. The camera will get you, but it doesn't matter. Approach the desk and talk to the clerk. Follow him to the back room and sedate him, then take his uniform.
Go outside the building and go to the garage entrance. Enter the security room and grab a videotape when the guards are not looking.
Go to the garage. Behind the van (*) you'll find a stylish costume - wear it. Watch out that nobody sees you changing. Now you have to leave your weapons. Get rid of the guns and approach the guards with the golden masks. One of them will scan you and you'll be free to enter the elevator. Go to the Top Floor.
When you're there, go up the hatch in the elevator and wait for one of the targets to enter. It might take several minutes, but will eventually happen. Use the fiber wire on him and take his golden-devil costume.
Now join the party. When the singer is on the stage, go to the office (*). Use the laptop there to know that there are two assassins tailing you in the building. One of them is the singer you've just passed by. Wait by the door until she's back, then kill her. You can take her stiletto and her gun if you want to.
Get back to the elevator and go down. Move to the second elevator. This one will take you to the basement. Now that's my kind of party!
... and hell.
Approach the devil bartender and you'll be challenger for a duel (he's the second assassin that's after you). Follow him to the Torture Chamber and face him. The weaponry is on the table where you finish talking. It's better to use a pistol on this one, as the SMG is not accurate enough. When you're done with him, take a key from his body. It will enable you to leave the room.
The last target is a girl who's performing at the stage with the pyrotechnics. You can set up a trap by messing up with the fireworks controls (behind the stage she's on). If you don't want to wait, just talk to her (you're wearing her man's costume) and she'll take you to a private room. In there you shouldn't have any problem killing her.
You can leave the level by using the van near the elevators in the garage. You might want to take your time to take back your suit, though.
The pyrotechnics panel.