This is a simple tutorial to help anybody struggling to achieve the "Silent Assassin" achievement.

1) Load the mission "Fight Night".
2) You will be told to assassinate a fighter, fighting in the ring.
3) Find the fighter who goes by the name "The Patriot".
4) Steal his outfit and wear it, you will be disguised.
5) Go to the ring.
6) Kill Sanchez in the ring with your bare hands.
7) Escape the area, picking up your weapons on the way out. It does not matter if you caught.
8) Watch the achievement pop up.

Here is a video tutorial: