The Wrecking Crew medal is obtained once a player kills 15 enemies with the Gravity Hammer without dying. It is available in most multiplayer gametypes and Campaign scoring modes and was revealed post-beta along with several new and updated medals in the 05/28/10 Bungie Weekly update.
It is represented by a steel 6-point star with a picture of a Gravity Hammer in the center of a purple circle. When someone attains and achieves this medal, the announcer will shout "Wrecking Crew!" to the player that received this medal.
One of the easiest ways to earn this badge is by using the camping method, simply hide near a popular place and kill people when they are in reach. This method has as a downside that people with (semi-)powerful long-range weapons would be able to kill you before you reach them.