The frost spell rotation is quite simple, especially while leveling.
Frostbolt frostbolt frostbolt (if the mob gets close, frost nova -> blink/strafe away to get some distance) frostbolt frostbolt (they should easily be dead by now).
If you get multiple mobs you can use Frostnova -> Cone of Cold. Or you can sheep/polymorph one and then move on from there.