Well, I ended up completing it. Got a base score of 123,???(something). Challenge round 2 bumped me up to 125,000. For the user above and anyone else that may stumble upon this thread looking for help, I’ll try my best to explain how I did it.
Their are 3 thigs that were pretty important for my success:
1) Finishing first
2) Making most of my air time
3) Spending a lot of time during the race smashing into other karts
I started by aiming the kart as far right as possible, with the intention of hitting the karts to my right. So as I kept smashing I took the path to the top right of the track; the space is small, making it the best place to start getting your smash points. If you find yourself getting ahead of the other karts, simply run into a tnt box to slow yourself down. After that…just try to keep playing that way. Make most of your air time when going down and collect as many coins as possible.
Hope that helps!
On another note, I got this same challenge for the timed race. Does anyone know how this is possible? Is time a factor? Do you get any type of bonus for avoiding all obstacles? There is this big block of ice near the end that seems unavoidable. I’ve stopped trying to complete the challenge for now, as it seems impossible, but I would like to hear from others that have attempted/completed that specific challenge.