the first thing that comes to my mind is to use contour points, and use just -for instance- the first two from the bottom for couples with the very same X coordinate… but it’s a bold assumption, I don’t think it’s what you are looking for, and it’s the countrary of “robust” 
Iso Curve_Re.gh (23.9 KB)
I think it would be much much more helpful if you could upload a few more surfaces with the final curve you would expect from each of them, just to give a bit more case scenarios