Samurai Wars now has a leaderboard. It uses an ELO rating system with a player average of 1500 points.
Although the ELO ranking itself is pretty straight forward, the Game Center app doesn’t really handle this kind of ranking ladder very well. There’s no way to display the current ranking, you can only see the highest ranking achieved (today, this week, or ever). I don’t know if Apple has any plans to support this, but still, the daily and weekly rankings are good enough approximations.