This cheevo will be awarded for playing any solo in any song and hitting more than 95% of the notes.
I got this on the 2nd song I played which was Nirvana - Heart Shaped Box and I was playing lead career path.
Because this was only the 2nd song I played since putting the game in the game treats me as a complete beginner and therefore once the solo came up there were very few notes to play and they were spaced out so it was very easy to hit 100%.
If the game is treating you like a very experienced player and your struggling to get this then you can always start a new game on a secondary save device like a usb and get the cheevo easily and then go back to your normal save after.