THPS 1+2:

To complete this challenge, land a 35,000 point combo off Venice Beach's All the Way Gap, using at least 1 Grind Trick.

I found that you need to take it kind of slow when first approaching the tables to manual on them or the challenge won't pop up. Remember to manual when coming off of them at the end as well or you won't have the opportunity to get enough points nor grind at the end.