You should be able to get this on any level of the saloon, but you'll definitely be able to get this on the Endless level, where the game only ends when you lose. After you lose, the bad robot will detonate a bomb that blows up the saloon. When you end up outside, look to your left and the piano and the piano playing robot will drop out of the sky, and it will start playing again. Aim your controller at the robot and shoot it in the head to get the trophy.