I did this song and found it easiest on Hall Of The Mountain Dude.
This is more of a way to save yourself time from playing the whole song.
I found using your score on the top right corner as benchmarks saves a lot of time; because Perfect, Awesome, Good, and OK all give certain scores it can be helpful. *
For the first part of Hall Of The Mountain Dude just after you do the last
and before you start the 3,3,3 notes your score should be 1,500,000+. The highest I ever managed to get my score before this part was 1,700,000 but when I got the achievement I had 1,600,000. If you score isn't 1,500,000+ you should probably restart as this is probably the easiest part of the song and you should try to get as high as possible to make up for errors later on.
After the 3,3,3 note part but before the first
your score should be 1,900,000+. You want to get at least 6/9 of the notes perfect. The highest I've ever gotten after this part was 2,000,000 but when I managed to get the achievement I had 1,980,000 at this part. If your score is a bit lower than 1,900,000 I'd suggest continuing but definitely restart if it's below 1,800,000.
About halfway through the song, there is this long break. At this part, you should have a score of 8,400,000+. I believe this part is the most important part to check your score as my highest score I've ever gotten was the same time that I got the achievement. I had a score of a bit over 8,700,000 at this part. At this point, you have to decide if it is worth continuing. My accuracy often dropped and I made a lot of mistakes during the second half of the song so if my score was below 8,400,000 I always restarted.
My final score was 12,520,075 with accuracy of 90% and Hit-ratio of 100%.
You also have to make a personal decision whether you want the 100% Hit-ratio and 90% accuracy or the 99% Hit-ratio and 91% accuracy. As missing 1-5 notes will still keep you at 99% Hit-ratio but you will have to make up for it with a higher accuracy.
(To get an A you need Accuracy + Hit-ratio = 190%)
*Do not use the multiplier, all the scores listed are without ever using the multiplier.