It's possible to do this achievement 2 completely different ways.
For this achievement, you'll have to complete the track "Pump it Up" (Hard) without braking ( or ), by only using gas/throttle.
Using the Phoenix 250cc, practice the track a few times until you remember it. Now go through the track, with your finger off the brake, and trying to complete it faultlessly. I recommend watching the video to see the best path and method for each obstacle. Just take your time and try to keep the bike very steady and flat to avoid wheelies.
Use the Scorpion 450cc as the bike is much more stable and reliable.
I was actually able to get the achievement having the throttle down the entire, while using the Scorpion and found it to be quite easy which is recommend this as an alternate method for those struggling. Just keep the gas pressed down the whole way through the track using the Scorpion, as it is quite stable, and try to get to the finish!