Pacifism often has the power to shut down your opponent. It’s a great card, because if you have one of these in your hand on turn 2, you can start locking up your opponent. Additionally, Pacifism is capable of essentially slaying creatures that cost 6 or more mana. And, for added help with Pacifism, I’d also recommend the card, Arrest.
In nearly every game of MTG, there are creature cards. These creature cards are essential with most people I've played in their strategy to win. Put out a few lock up cards, and your opponent will probably have his offense shut down.