This minigame, like several, start off pretty easy at first, but in my opinion, get ridiculously hard. I've been playing guitar for nearly 30 years, and playig this legit, i could barely break 2 million before the ducks started coming down too fast. So here are two tips to help you crush this minigame.
1. Abuse the pause feature. Every time you pause, you get a countdown of 3 seconds, so you can reposition your hands and see what is coming down to hit it. Towards the end, I was hitting pause after hitting every note...and even then i barely had time to hit the note and repause.
2. Unless you own a guitar with perfect/very good intonation, chances are VERY high that the guitar intonation will affect your ability to play this game at the higher frets especially on your lower strings past frets 15 or so. I am not sure what ubisoft was thinking, since most players rarely use those frets in real life anyway. What i found happening more often than not, was that I would fret at 17 and the game would register it as hitting fret 18 causing a miss due to my intonation being off and my guitar going slightly sharp. To avoid this, simply find the note you are suppose to play, listen to the note, and match it on another string. For example, if you are supposed to hit fret 19 on the red string, just hit fret 12 on the yellow string instead.
Don't worry if you think the game is going fast and you are still at like only..2 million or so. Once you get to the later levels, the score starts to skyrocket as hitting individual notes later will easily yield huge scores that boost your score in no time.