The easiest way would really be to take a 1000-2000 grit sand paper and VERY, lightly, with virtually no force rub it against the scuff. If it still doesn't come off, try a 400 grit.

Have you tried using a polish as I suggested before?

Rubbing compound sounds like it would work for this

FWIW,

I sprayed my lip w/ OEM Bumper Trim paint b4 installing. Anytime I get any small scratched/damage to it, all I have to do is light sanding if required, and give it a small spray of the trim paint, and it's as good as new.

I am going to have to try this on my car