This achievement was one of the few I actually had to work for. It's a very complex solo, and one of the most difficult in the game. There's no surefire way I can tell you how to get it, only tips.
First off, practice it on all speeds. What I did to help make sure I could get it down was to FC the solo 3 times in a row on 50% Speed, then do the same on 60% speed etc.
80% Speed was a bit tricky because now, I had to actually play the solo like it was because it's quite hard to abuse the timing window on the zig-zags. One thing you should notice is that the BYG's, OBR's, the OBY's, and the YRG's in the zig-zags are faster than the other parts, so you'll need to hit those at a faster speed than the other parts. When you can consistently hit the zig-zags, try playing the song out in Free Play or Tour and see if you can get the solo.
The solo outside the zig-zags is much much easier, so if you hit the 2nd zig-zag pattern, it should be very easy to get the rest of the solo, as there's nothing hard outside of a couple quick strummed Greens.
Good luck, this one took me quite a bit of grinding to do.
EDIT 1: I tried it on my alternate account, and unless I'm missing something, you can not do it in practice. And you don't have to FC (100% with your combo not broken at all) the song. You can overstrum as much as you want, as long as you 100% the solo in Free Play or Tour.
EDIT 2: LordMathias:
"Here's a slight, albeit vague, tip: try playing it using different methods. Of course, practice at lower speeds first, but if the fingering method you're trying to use isn't working, try something else. I went from missing in the second RBO part (because my fret pinky sucks) to 100% in practice using tapping in a matter of 20 minutes, and then second try in free play *pop*. If I wasn't lazy I'd post up a video, but yeah."