This achievement requires you to get 100% on 20 different songs on expert. Meaning of course you have to hit every single note in a song. You may overstrum as long as you don't miss any notes, the combos don't matter.
If you plan to do this achievement the 'legit' way, check out the other other guides as they have good song choices for you to get 100% on.
Ill give you a few that I found easiest to get 100% on to get you started:
Slow Ride, Story of My Life, Mississippi Queen, When You Were Young, My Name Is Jonas, Closer, Radio Song, Ruby.
EDIT: BraverLeonheart in the comments section says that this doesn't have to be done all on the same band and can be unlocked across multiple bands.
For people who can't or can't be bothered to 100% 20 songs on expert:
*First, go to your System Settings on the Xbox Home, then select Storage, scroll down to your hard drive and press Y, then Clear System Cache and press Yes. (This will delete any updates that have been downloaded to your drive)
EDIT: Some say that there is no need to clear the cache to do this glitch, even I have seen the glitch work online... but it doesn't always work. So its up to you, try it online if you like.
Now boot up Guitar Hero (Press "No" when it asks to download the update) and go to Options, Cheats, and then Enter Cheat.
Enter this cheat: (This is the Unlock Everything cheat, no sound plays when you enter these chords)(Each letter represents which colour to hold down on your controller, then strum whilst holding the colours)
Once you have done that go to Quickplay and Expert, Bonus songs and Through The Fire and Flames, then just fail the song.
Now go back to the Cheat menu and re-enter the cheat, fail Through The Fire and Flames again and then... Achievement Unlocked.
Also if you go to the Store in the game you will unlock 4 other achievements if you dont have them already.
Please don't abuse the voting system. If you follow the guide it will get you the achievement. Blah blah blah, this glitch ruins the TA score for people who did it legit... Yes, it does. If you did it legit, congrats, go post your scores over on scorehero.com and see how good you actually are.