I like Elephant's idea. I think AE does it that way. My group uses an Opposed Roll (at least, that's how we did it last session), which is actually the AE Tumbling rule, but it worked pretty well. It still gives the edge to the casters except where they're outmatched by their melee combatants.
My personal sensibility is that in situations where two characters are actively working against each other, Opposed Rolls make more sense than set DCs. With set DCs things are usually impossible at low levels and impossible to fail at high levels, and neither of those extremes are particularly exciting.