I hate to advertise other servers but I played a server (before this one) called CNATION. They had mob arena as well and the way that "score board" thing worked was in the spectators area it would have a list showing the names and amount of kills each of them had. Spectators were able to see it and it would work as a leaderboard system. Names would go up or down depending on the kills they obtained. And yeah that was pretty much it...
I don't know if this server still have it but this is what I remember.