Underdog Trophy in Trials Fusion
Beat a score of 40,000 on 'Final Frontier' whilst riding the Roach with the Frontier BodyKit
How to unlock Underdog
This is probably the hardest trophy in this DLC.
The first thing you need to do is equip Frontier Bodykit on your Roach in the Garage. It's free and comes with the DLC.
You are then required to perform FMX tricks on Final Frontier track, using the Roach. FMX scoring rules apply. In other words, you want to perform as many unique tricks as possible, and complete the run without faulting. You also need to build up your multiplier by doing tricks continuously without screwing up. Assuming you did all that you'll end up getting over 10,000 bonus points at the end. One way or the other, it will definitely take you a number of tries.
Here's a video by Maka: