Pretty much exactly what it says on the tin. Impossible to do by yourself though (I think), so getting band mates is essential. Have some local players with you, or look on Xbox Live. It seems that the maximum number of players you can have at a time is four (that count towards your score as All-instruments mode wont count vocals). The harder the song, the longer the song, and the harder difficulties you can play, and the better you can coordinate overdrive the higher you can possibly score. See scoring system at the bottom.
You can use any song you want, but you must not fail out of it to get the achievement.
For the sake of curiosity I got this with an Expert Drums, Vocals, Guitar and an Easy Keys player on "Smoke on the Water". Although one of the guys online said he already got this with a full Hard band on "Llama".
The following are the longest songs available on RB3 as downloadable songs. These will likely help you get it even if you play at a lower difficulty than Expert:
"The Who Super Bowl S-mashup" by The Who
"Jailbreak" (Live) by AC/DC (from AC/DC Live)
"The Camera Eye" by Rush
"I Heard it Through the Grapevine" by Creedence Clearwater Revival
"...And Justice For All" by Metallica
"Green Grass and High Tides" by The Outlaws (from Rock Band 1)
"Let There Be Rock" (Live) - AC/DC (from AC/DC Live)
"Homecoming" - Green Day (from Green Day)
"Jesus of Suburbia" - Green Day (from Green Day)
"High Voltage" (Live) - AC/DC (from AC/DC Live)
"Descend into the Eternal Pits of Possession" by The Project Hate MCMXCIX (from Rock Band Network)
"I MAED A GAM3 W1TH Z0MB1ES 1NIT!!1!" by James Silva (from Rock Band Network)
The 'goal' is situated in Rock Band Mastery - Score - Millionaire Club
10 - x2 (1 phrase)
20 - x3 (2 phrases)
30 - x4 (3 phrases)
40 - x5 (Bass and Pro-Bass only)
50 - x6 (Bass and Pro-Bass only)
Harmonies/Vocals (Each Awesome phrase on difficulties):
Easy: 200 (20 for each harmony part)
Medium: 400 (40 for each harmony part)
Hard: 800 (80 for each harmony part)
Expert: 1000 (100 for each harmony part)
25 points a note. Chords are 25 times the number of notes in the chord but only counts as one note to the streak.
Pro is the same but at 60 points a note.
25 points a note. Two notes played together are separate notes for the streak.
Pro is 30 points a note, except in the case where if you are supposed to play a cymbal, and you haven't selected it in settings as a cymbal you choose to play (so it would be charted on the track as a "normal" note) then those are 25 points.
I.E. If you want a maximum score on Pro Drums, you need to play with all three cymbals.
For Big Rock Endings try and play randomly and as fast as possible. I noticed that for Guitar, Bass and (Pro) Keys that if you continuously play the same notebutton over and over again the score doesn't go wildly high as it would as if you were playing different notes. Try playing Glissandos if you are on Keys or have Solo Buttons on the guitar.
This isn't so much of a problem for Drums and Pro Guitar/Bass. Just keep hitting or strumming away.
Just be sure to nail the final notes!
For tambourine sections on just keep tapping away on the Microphone or the A button on the controller. The points for tambourine sections are only given as a bonus at the end of the section rather than points for every note. The score is based on how many notes you had to hit and how many you did hit. It is similar with Guitar and Drum solos, but the bonus is on top of the normal note scoring. For Drum solos you do not get Drum fills or any chances to activate overdrive, be careful.
Overdrive gives the band a 2x multiplier for the band from each instrument that activates it.