Well, first, you aren't going to care about levels much. Winning a fight is determined by trophy penalties, skills and refinements. Levels are for bragging, and not much else.
Second, as you can see, that is a big difficult messy problem, trying to figure out a fair handicap for all those complications.
Third, I question the point. If it's equally fair (by which I presume, both sides have an equal opportunity to win), you have essentially turned studio fights into a coin flip. Right now the winners are the studios consistently building up incremental advantages in skills and refinements... Kind of defeats the point if all of a players work is equalized away.