Go to the Rock Ridge freestyle track. Go to the left until you notice the chair lift. Head downhill to the lower tower. Look for the stone ramp to the right (while still facing downhill). Get your speed up to about 45 mph, preload, and jump to the tower and stop on top of it. You must now travel up to the higher tower riding on the cables. When you reach the other side, will get the "Lift Tower Challenge" trophy and unlock Elrod the Viking.