A couple of notes. First off, when you get to the mission to shoot the guy's leg in order to keep him alive for questioning. Being that his legs (as well as all the other sticks) were so thin, you had to aim exactly on that little line that is his leg.
Second, grammar. Only one mistake, but I just had to point it out. "You're are best sniper, Johnny".
And now for things that would make the game more difficult. One idea would be to add more people in the scene. It's far too easy to shoot your target when he's the only one there. Throw in a crowd of people, thus making it harder to figure out which one to shoot.
Something else to add would be a time limit. Y'know, whereas if you don't kill your target in time he could simply walk away and you would fail the mission.
'tis a good, simple game. Just needs some improvement.