For this play style, you need to get several kills using a Katana. You can pick up a katana during the Chinese New Year section of Hunter and Hunted or the Penthouse of Blackwater Park. The katana in Chinese New Year is located by a stall. This stall belongs to the man who is killed and pushed into an open ditch by one of Wade's men. If you are having trouble find thing then there are many of them in the Penthouse. The easieast one is found by exiting the elevator shaft into the penthouse and then going left twice. You should see Samurai armor to the right and a katana on a table to the left.
Chinese New Year is a better option because there are so many people there but it's a little harder to find. Either way, grab the sword and kill as many enemies as you can. It shouldn't take many kills (probably less than 10) to unlock.
