Requires the Jedi Temple DLC.
Simply beat this bonus mission on the Sith Master difficulty. To start this mission, you must make a new game. It should then allow you to choose either the Jedi Temple or the normal game.
There is a glitch that makes this achievement stupid easy to get. Unlike the main storyline difficulty achievements, there isn't a check to see if you changed the difficulty. Because of this, you can start playing at the Apprentice difficulty and switch to Sith Master via the Options menu just before beating the Sith Warrior.
Unfortunately, this achievement doesn't unlock the lower achievements.
Note: There is a possibility that you will need to reach the 3rd part of the battle (the part after he was throwing the large column pieces at you) change the difficulty to the desired dificulty, allow yourself to die, change the difficulty back to apprentice, take his health down until you see the prompt to start the quick time event, change the difficulty back, and then beat him. I had trouble getting it to unlock if I didn't do this.