S7 one is probably the easiest to beat when you know what to do. My best time is 2:07.901, a full 11 seconds inside the time needed for Bronze, so I know something about this (though others who frequent this board are even faster).
The main technique you need is to hold the higher gears. The Griffith has such a torquey motor and a predilection for oversteer you need to control the amount of power going to the back wheels.
Either use MT, or use the L2+R2 trick to hold the current gear. There is no need to use anything lower than 3rd. Even then you will need to use part-throttle on the corners.
The other thing is to learn this b*****d track. It's long and sinuous, but once you hit your braking points and apexes, combined with power sensitivity the time will come easily.
{edit} Just about everything I've said here also applies to S8