For this Trophy/Achievement:
Complete the Ice Deadly Descent WITHOUT using ice axes, when selecting the Deadly Descent in the Siberia select your favorite rider (if given the option) then select none in your gear, you should receive a notice (if you didnt turn the notices off already) warning you that you have a 4% chance of survival.
For this one I can only say practice makes perfect. Personally I started out going to the right, use the rocks on the sides to slow you down, and use rails! You will try this many times before getting it don't forget you got rewinds, try to save them as long as you can.
I believe it took me at least 12 tries to get it.