I struggled so much with getting Gold on Auchindoun Coaster Reverse. Took me about 2 hours to figure out something that worked for me. You know how you're always like 1-3 seconds short every time and don't really know where you can actually gain speed to make up the time? What worked for me is by gaining speed right at the very start of the race and maintaining that speed throughout the rest of the course.
As soon as the timer ticks down. Back to back spam Surge Forward twice even before your wings unfold. This will take you through the first 2 rings. Immediately after hit Skyward Ascent when you grab the first green Vigor bubble in the second ring. This should give you enough speed to maintain throughout the rest of the race.
Now that you have your speed, starting roughly around the 10th ring you WILL need to ground skim to get extra vigor bubbles so try to stay as low as you possibly can. Try to ground skim as much as possible throughout the rest of the race and just spam your Surge Forward.
This method is what got me Gold. Hope this helps!