This achievement can be a pain if you are not to good at Expert difficulty, especially on Guitar as this is the easiest to get 1,000,000 on since a lot of songs contain a lot of chords. To get this achievement you need to score 1,000,000 points in a song by yourself in quickplay+ Power Challenge. The easiest song (so far) that I have found is “Aqualung” by Jethro Tull, it’s a long song with lots of chords and the start and end of it are relatively slow. So select this song and go to Star Challenges and select the first option which is Power Challenge. It will now ask you to choose your powers (chose wisely as you can only have 2), I find that Star Power Nova+ & Multiplier Extender+ are the best as Multiplier Extender+ boost you to 6x multiplier instead of the usual 4x, then when you use Star Power your Multiplier turns to 36x thanks to Star Power Nova. The best thing to do in this song is collect the first 3 Star Powers and hold on to them until the long stream of chords that are nice and slow, use star power for the first time here. By the time Star Power runs out you should be well over half way there. From now on use Star Power where you think is best and when you have a 6x multiplier so you can raise it to 36x. If all has gone well you should end up with well over 1,000,000.
You can also earn this by using the same tactics above using Star Power Nova and Amplifier, this gives you the chance to play it on easier difficulties (but a better run may be needed).
You can also do this on other songs that are big chord fests from imports or DLC but this song is one of the only on disc songs that is very easy to get it on.