Downhill is the 8th Advanced floor in Monkey Ball and Super Monkey Ball, as well as the 7th Advanced stage in Super Monkey Ball Deluxe. It also appears as the 47th Ultimate stage with continues and the 67th without continues.
Downhill is a stage with a large, descending ramp leading to the goal. There are walls covering half the ramp that alternate and are spaced out far enough to allow your monkey to navigate around them without falling out.
Roll down as fast as you can down the ramp until you reach the goal. Try not to go too fast because your handling could have your monkey hit a wall and fall out.