If you're willing to wait, the last Survival mission (Home Security), which is in Clearance Level 5 is ridiculously easily to get Gold on. There aren't any batteries, but keeping your shield should be very easy since checkpoints are more frequent to make up for this.
Midway through the track, there is a giant banked turn with a drift strip. Instead of going on the bank, stay on the inside of the turn to save time.
To make this even easier, you can enter the Unlimited Turbo code and keep tapping X during the race to boost. The code is 721953. If you want a homing laser at the beginning of the race, you can enter 123456 as well.