Rock Band 3

While this may seem daunting it is quite easy and just takes patience.
The goal is to actually get a score of 12,345,678 over as many songs as it takes... Not just one.

The key thing to note is the score counts for your best score on the song with only 1 instrument and difficulty; for example if you play 'Combat Baby' by Metric with Guitar on Easy and score 1,000,000 and then play if for Bass on Hard and get 500,000 only your score for Guitar on Easy would count.

Obviously with the the longer you play or the bigger your library the easier it will be. It can be tracked through your career goals in the menu aswell.

Unfortunately, this achievement isn't very well-defined. I think that the above solution is inaccurate, however. I believe that this achievement only tallies *one score per song*, whether that be on expert guitar, medium drums, easy vocals, whatever. Whichever score is highest. Evidence:

1. I have the "immortal" goals (5,000,000 per instrument) for three instruments (bass, vocals, and guitar), and am over halfway on drums. I've probably put over 20,000,000 points on the board. Yet, I only show 10,860,000 for this achievement.

2. I just selected the goal (under "career"), and chose a song that I had only ever beaten on expert bass and expert vocals. I played medium drums (5-starring the song). At the end, the game said "you did not make any progress toward the Alex's Luggage Combination Goal", and back at the goal screen it read the exact same score as before.

Due to these two things, I sadly think that the way this is really tallied is top score per song, period. Doesn't matter what instrument or difficulty. On a brighter note, it does tally band scores for a song...so if you play with three other people you will quickly get this.

Of course, everyone in my town has the flu right now, but you may have more luck with your friends than I. Anyway, hope this helps!

EDIT: DLC also counts toward this (thanks Troodon9999), so if you have a fair cache of DLC (or have imported RB2 songs) it's still quite easy to nab this solo.

Play the game. As far as I can tell the game tracks your highest score for each track played, regardless of instrumentation, number of players of difficulty. DLC is counted.

I played for about a week and just unlocked this out of the blue. Just make the most of this incredible game and it'll pop when you least expect it.

I have read the other Guides and couldn't figure out how to improve on my score or where to check my score so I am writing a guide for both of those tasks.

Getting this achievement is easy, but it will take time. I had already played several songs before I decided to go for this cheevo, so I'm not sure how long it might take if you start from Zero points.

You can play this on "no fail" but it helps to play at a level you are comfortable with.
It also helps to have a large library to work with. You can add songs from your other Rock Band Games if you need additional Tracks. Rock Band, 2, Classic Rock, Country, Metal, Track Pack, AC/DC, Green Day, and LEGO to name a few

You need to get a total of 12,345,678 for your best performance across all of your songs. Multiple instruments will not help because only your best instrument performance is counted. If you play a song on guitar, then play the SAME SONG on drums, only your best performance will be counted. So pick your favorite instrument and stick to it.

TO CHECK YOUR CURRENT TOTAL

Select Career then My Goals, Rock Band, Epic Rock Band 3 then scroll over to the third box.
It will show your total below "Earn a career score of 12,345,678"

TO EARN THIS ACHIEVEMENT

Select Play Now, Quick Play, Choose Songs

That will open your song library. Press select on the controller and choose Sorted by (the first one) then scroll down to choose Play Count.

If you start with the songs you have played 0 times you will be sure to add score to your total. Just keep playing songs you have never played on a level you are comfy with until the achievement pops.

If you have played every song once but it still has not popped, go back through the songs and play the songs that only have 3 stars and try to improve your score.
If that still doesn't work try adding some more songs to your library.