Sorry, but you picked a REALLY tough shot to roto. Blurs are exceedingly difficult. You're just going to have to keep plugging away at it. No magic bullets on this one, I'm afraid.
Just in case you get sick & tired of the roto work, do you have a Plan B to pull this off?
This is the answer I was afraid of... I guess I'm gonna have to keep masking it all frame by frame, yey! And no, unfortunately I have no plan B, some of the shots simply do not have alternatives. The only other way is not to change BG in them, but when you compare those to completed shots with matte painted cool sky, they look really boring and out of place...
it could be a very tricky shot but can we see an example? this would help to examine the problem and the solution.
with fast movements I found Rotobrush to do extremely well (much better than very slow movements)
see if you are using motion blur 16 samples per frame, High Shutter angle and High Quality checked on, and combined with refine edge (via Rotobrush) in my experience I got most of the smear, what is left I can do with brushing frame by frame with the paint tool over the Rotobrush either on alpha or even painting some of the skin back in RGB (if the background is more prominent in the smear than the skin color) but again without seeing your footage it's shooting in the dark (and really fast!)