This requires you and a buddy to consecutively hit 1000 notes in a co-op song. This one is a long haul, as 1000 notes even in co-op is a long time to play perfectly. A tip: don't worry about star power or any other distractions, just focus on playing all your notes. There are a few songs that can be played on Medium for this, but the majority of them are on Hard.
Good Luck!
EDIT:
Famous Last Words (DLC). Easy Guitar/Medium Bass, 1246 notes from 100%. (Confirmed by Codehead UK and spammy2505 - thanks!)
Surrender Medium/Medium confirmed by OFFICER ZERO.