Should be called the ONE OF TWO LONG ROADS AHEAD ACHIEVEMENT.
One however is slightly longer. The first road is legit one, without using the unlock everything shortcut.
For this you will need to :
1. 5-star all solo careers including all of co-op
2. Buy everything from Guitar Center, the store.
3. Complete 100 ranked SONGS on xbox live.
Before you freak out let me explain. The only reason you need to 5-star all the careers including co-op is because you are not using the cheat. That means that certain guitars won't be available till you do this.
Next, you may have noticed how I said complete 100 songs rather then matches. I say this from personal experience. I played around 50 matches yet much of that was boosting towards 500 wins, playing best of 7 battle matches. I put in the code as a shot in the dark, played a song and it popped up at the end. Pleasant surprise would be an understatement.
Your other option is the one I chose. You will need to:
1. Complete all difficulties of the solo career.
2. Complete 100 ranked songs on xbox live.
3. Put in the unlock everything cheat.
4. Play a song in career (you can fail if it pleases you).
If you put the cheat in before you complete steps 1 or 2 then you will need to complete steps 3 and 4 a second time.
(GR_BO) (GRYB_) (GRY_O) (G_BYO) (GRYB_) (_RYBO) (GRYB_) (G_YBO) (GRYB_) (GRY_O) (GRY_O) (GRYB_) (GRY_O)
I hope this helps clear up confusion and saves you a lot of time.