To start the mission, go to SINGLE PLAYER ->CHALLENGE ->ASSASSINATE THE FÜHRER

If you want to get the achievement for a testicle shot and you kill Hitler with an other shot, press ESC before the bullet cam ends and restart the last checkpoint.

The Führer is running when you can shoot him in the balls, so you should shoot when the crosshair is in front of the target, the bullet needs some time to travel while the enemy still moves forward.
You can follow him with the crosshairs and overtake him and then shoot in front of him, or you just aim at a point where he is running to and shoot when he is maybe half a meter away from this point.

Easy way to get Target Führer Achievement:
While Hitler hides behind the truck, you can see his heel. If you shoot his heel, he stumbles and you can see his buttocks to the left of the wheel. At this moment you can hit his testicles with another shot.