STEEP

All you'll need to do to unlock this is complete all 21 Winterfest challenges, none of which are very difficult at all. Boss challenges are every fifth challenge, except for this one, which unlocks immediately after beating the fourth boss challenge.

The Beast is the final boss of the Winterfest DLC (21st challenge) and beating him can be difficult, at first. But he is actually not that fast. If you ride the part before the big drop as fast as possible (no sharp turns, just ride it out), you allow yourself room to mess up later on and still beat him.
This video shows that you don't need to ride a flawless race to defeat The Beast. I mess up big time by falling hard and riding in the wrong direction, but I still manage to be faster in the end. The video also shows some relative "safe" routes to take.