Thanks for the guide.
A few additional tips:
1) For "A Looong Waaay Down", like it has been said, make sure you activate boost before going off the waterfall and JUMP, and keep boost activated the whole time (make sure you have a full boost bar). You need to land near the two boats that you must go through, otherwise the achievement will not pop. Also it pops instantly, you dont need to finish the level.
2) For "Turbo Starter," you do not need to worry about the throttle at all. Just tap the screen right after the 3, 2, 1, and the GO. It instantly gives you full throttle and boost and the achievement. This was simple, not sure why the guide says it is tricky.
3) You would have to be horrible at this game to take 8 hours to complete it. I did it in 3 hours. I personally would adjust the completion time for the guide to 2-5 hours, depending only on skill. The difficulty is most certainly not a 4 IMO. I would set it at a 3/10.
4) It may cut down on you time (which is what I did) if you win the Expert Boat gold trophy in Race Mode on a certain map, then switch out to Time Attack mode immediately after on the same map while you are still familiar with the shortcuts and red boost locations. After getting gold in the race, I only had to play each level once in Time attack to beat the par times (with a few exceptions).