Who is this mystery man? Find out and kill him for a 1.12 point modifier.
Although the Challenges are presented so that it looks like you can complete this one prior to the final one, the reality is that you cannot. To cause this Mystery Assassin to appear you have to have completed all of the other Challenges except this one, after which point all you need to do is target the Mystery Man through the window and shoot him once to unlock this Challenge.
Good on ya mates! You have unlocked every bloody challenge in the game! Well done! Now all that is left is to earn the required money to complete all of the remaining Unlocks, which sadly might require a fair but of shooting, but hey, at least you have all of these multipliers unlocked, right?
Assassinate the Mystery Assassin