[Lazortag]

Here's what I came up with (using an exponential function): http://www.wolframalpha.com/input/?i=100-100%284%29^%28-0.01x%29+from+0+to+400
(this has an asymptote at y = 100)

You'd need to add constants that factor resist modifiers (like with lifetaps) and the ratio of the two players' levels (ratio instead of difference since I'd think a level 10 should intuitively stomp a level 2 much easier than a level 50 stomps a level 42). This is a little bottom-heavy also (having 50 MR gives you roughly 50% resists with this which is kind of ridiculous) but I just wanted to have fun like everyone else.