In the game mode guardian you have to assassinate the other teams leader. Unlike Halo there is no little arrow/waypoint that shows you where they are. But if you do this one very simple thing finding the guardian will be a sinch. To see a star over the guardians head you simply hold the left top button of the controller *labled LB* to bring up the stat com I belive it's called and look around until you see the bule or red star above thier head, and that means they are in that direction!
Hope this wasn't too long!