Ambassador is for spies that couldn't get a spot on the sniper squad because of class limits. If they are stupid and have razorback on, then the spy should just get his revolver out.
A half second after the first shot hits the sniper, the sniper reacts.
By the time he is fully aware of a spy, you get your second shot off.
Now he is at 5 health. After the second shot is fired, he pressed right click to unzoom. Then he is dead.