I read the Ichi the Killer manga recently (as evidenced by my avatar), and in the series are quite a few torture methods which are so horrible, they make my stomach churn just thinking about them. The one I would do to an enemy may be the worst.
Penis mutilation.
Cut the penis in half, then bandage the two sides so that the guy doesn't die of blood loss and so that the penis won't grow back together. Stick pins in the penis and give the guy some piercings. Just go at it with a knife while the guy screams, and always bandage him up afterwards, so he'll think that his life is ruined, even if he manages to escape. Of course, he wouldn't ever escape, but I wouldn't tell him that.
So many things you could do. It's frightening.
If it's a girl, then, well, I guess I'd do a little research on vaginal mutilation.