Total star count of deck. From what I have heard and seen you are matched against players with + or - 5* from your current deck. So the popular 16* decks are fighter players with 11* to 21*. A good stratagie is to place many low star card up to 9 or 10 to go against high star count opponents for the higher bonus points awarded for victories. The challenge is your opp. has higher and "more power" cards then you. All about strategy.