This achievement requires completing the 31 challenges on Sierra Rivera, which is the third mountain.
The hardest challenge for each trail has both a time and a crash requirement. The other challenges are much easier since you can take your time for the "x crashes or fewer" challenges and you can blitz checkpoints and crash often for the challenges with a time requirement.
Here are my successful runs on the hard challenges which show workable paths through the trails. These runs are far from optimal but were sufficient to complete the challenges.
Trail 1 in 2:10 or less with fewer than 7 crashes (Trailblazer)
Trail 2 in 2:20 or less with fewer than 7 crashes (Boar)
Trail 3 in 2:00 or less with fewer than 5 crashes (Geronimo)
Trail 4 in 2:35 or less with fewer than 8 crashes (Grasshopper)