At the point you've gotten all the other achievements, you'll aready have done 35 solo songs, 35 duet songs, 2 songs for gig sessions, 1 for team play, 1 for versus, and 1 for You're On Your Own. That leaves 25 songs left to sing, or less if you actually played more than the minimum.
The easiest way to do this with the least amount of fuss is to enter solo gig mode and set up a gig of five songs. By now you've discovered that you don't actually need to do any singing to get credit for a song, so catch up on your Jane Austen or Dan Brown while you wait & just set up a new gig of five songs each every 10-15 minutes or so. Do that five times and you'll have your achievement.