No one is invincible. Everyone has a weakness. It can either be well disguised or openly seen. What matters is that you find your enemy's weakness before he or she finds yours.
Finding one's weakness is easier said than done however. Everyone develops a psychological defense as they grow up to cover-up their weak points. It would take sly tactics and a strong set of nerves to break that defense.
Carefully examining the unconscious signals sent by your enemy is vital in finding a weak point. Pay attention to your victim's actions and see if anything can be used to your advantage. If your foe is an emotionless rock and doesn't give anything away, pretend to open up to him or her and hope that he or she opens up to you. Remember that most weaknesses develop during childhood, so try to go as far back as possible when talking to your victim.
When dealing with a group of people, make sure to go for the weakest member first, or as most call it: the weak link. Again, search for any insecurities or uncontrollable fear, and take advantage of it.
The trickery an art dealer named Joseph Duveen performed is a perfect example for this law. There was this humble woman named Arabella Huntington who was branded as a gold digger for marrying a very wealthy man. Realizing her insecurity for being wrongly perceived by society, he began courting her and getting to know her for who she really is. She loved the way Duveen treated her, so when her husband eventually passed away and left her all his riches, she used the money to buy his paintings. Duveen's plan worked perfectly. Instead of simply selling her his paintings right away, he first found her soft spot then capitalized on it.
Now before you run off and begin looking for your nemesis' thumbscrew, remember that you're doing this for power, not for the pleasure of toying with your enemy. Danger lies in losing control when pushing the wrong buttons.
So... What's your weakness? I'll tell you mine if you tell me yours!