From your heading I gather the mountain in your story is covered in snow.
If the slope allows it, then I'd skid down on my behind. Grab something like a suitably formed stone to be able to perform a self-arrest.
Any kind of skis you could expect to build up there would perform far worse.
The technique @helm recommends is (at least in this neck of the woods) called a "Glissade". It is essentially "a method of descending a steep snow-covered slope via a controlled slide on one's feet or buttocks". I have heard of people using other items to assist in a glissade by using them as a sort of sled, a backpack or as in Ed Abbey's Desert Solitaire; a ...