Method #1
Kill Pierce by making an another assassin kill him.
(If the Assassin fires his railgun it will cause mass panic in the Penthouse but they wont suspect it was you.)

Method #2
Kill Pierce by planting a C4 on his helicopter and blowing it up when he attempts to evacuate the penthouse.
(Pierce will evacute the penthouse in 7:30 minutes if the assassin kills his bodydouble)

Method #3
Kill Pierce by pushing him off the penthouse while he's on a phone call.

Method #4
Kill Pierce sliently while he's in his bedroom

Method #5
Kill Brody by shooting the glass jacuzzi.

Method #6
Kill Brody by shooting the glass railing and watch him fall to his death
(Shoot the glass railing in his private balcony before Brody arrives and leans on it)

Method #7
Kill Brody with a knife while he's in the bathroom

Method #8
Snipe Brody and Pierce slienty without causing alert.