Expert challenges are unlocked by completing each trail once to unlock the beginner challenges, then completing the beginner challenges.
Each of the four Eldfjall trails have two expert challenges. The first only has a time requirement and the second has a less strict time requirement and
a crash requirement. The second challenge on trails 1, 3, and 4 require the use of a specific bike (Pacebreaker, Javelin, and Boar respectively).
The first challenges are made easier by the fact that you can blitz checkpoints and crash any number of times. The second challenges require a little more precision but allow for a fairly reasonable amount of crashes.
The key to these challenges is finding shortcuts on the trails allowing you to shave off as much time as possible. Here are some zero crash runs I recorded to show the paths to use:Trail 1: The Rift
Challenge 1: Finish in 1:55 or less
Challenge 2: Finish in 2:10 or less with 14 crashes or fewer (Pacebreaker)Trail 2: Twisting Vales
Challenge 1: Finish in 2:35 or less
Challenge 2: Finish in 2:50 or less with 14 crashes or fewerTrail 3: Great Ocean Crest
Challenge 1: Finish in 2:15 or less
Challenge 2: Finish in 2:30 or less with 21 crashes or fewer (Javelin)Trail 4: Scorching Descent
Challenge 1: Finish in 2:50 or less
Challenge 2: Finish in 3:05 or less with 14 crashes or fewer (Boar)