I was on my own so I set up the full band, with 3 guest accounts and my own account logged as the guitarist as it was easier to strum one-handed (ho ho) and keep the other 'players' in it.
Then I put everyone on Beginner and basically embarrassed myself by jumping around and doing just enough to ensure no one quit out whilst strumming the guitar as much as possible to ensure that scored highest.
Basically this involved having a guitar slung around my waist, the bass propped up on the sofa, and the drum kit next to me.
Then I'd sing, occasionally strum bass and guitar together, or guitar and pat the drums together, whenever one of them went into the red.
I probably looked like an idiot, but it worked!
I also did this for the Vocalist's Coat-tails achievement on Guitar Hero World Tour except my account was logged into the mic obviously.