For those who may look at this and think its tricky, its really not!
I just ran around New York assassinating Guards (Red) and Assassin Gangs (Orange) until it popped. I didn't count exactly how many i killed but it was around 30. It must of Taken me no longer than 5 minutes. Just bare in mind that guards will detect you immediately as your wanted level goes up straight away once the cheat is activated.