Yeah dunno tricky...
The best i can come with is some sort of a point system, where you gain points when kill a player proportional to the point difference (if you kill weak chars, you get almost nothing), and lose points proportional to the point difference when killed (if you are killed by weak char, you lose TONS). Reward positions could be associated with a higher loss - for instance, if one gets a reward by killing top 1-5, top 1-5 will lose more when killed to make sure it will take a while for them to climb back.
Still there are ways to abuse it (top folks could abuse it between them, for example) but at least it requires more effort.