This is really easy, but just to make sure people don't miss it I’ll spell it out. *Almost slight spoiler*
You get a slowmo scene here when you find Castro in his bed room with a lady friend. He'll use her as a shield, but even on Veteran the slowmo makes it easy enough to get the head shot on your first try. You'll know you did it right when you get a "bullet cam" that follows the bullet right into Castros head. *BOOM! Headshot!*