Thought I would make a video for how to do this easily since this achievement is a tad tricky, but not difficult. But the lack of the method means a video is in order to help out.
The achievement is story related and located on the prison level.
The achievement basically means you have NO CHOICE but to stealth kill sinister but this is a little bit tricky took me about 5 goes because I was careless with the guards patrol patterns.
Anyway here is my video on what should be a flawless method.
Also you do not have to use the sword since he seems to equip it when you kill sinister anyway so no worries about what weapon to carry.