D - Demoralized
Even the strongest of foes, or marines, which for the Strawhats is the same thing, can be demoralized. All they have to do is be stupid enough to harm or catch on of the people Luffy cares about. It could be a friend, a crewmate, or a person who refuses to be a friend (a.k.a. Law). Either way, Luffy and the rest of his crew is sure to run to there rescue.
Sometimes, people thing they are very clever, and take a hostage to ensure there safety. This always backfires for them.
There are many instances of this, one of the most well know being Enies Lobby. But the end is always the same. The enemy stronghold, or ship, in shambles.
Afterwards Luffy always has a large party, which may seem like another way to rub it in the enemies faces, further demoralizing them. (However, the captain of the Strawhats is not considered smart enough to thing of it that way.)