Select the "Number Of The Beast" song and tape the whammy bar down. Leave it there for the entire song. Even if it is not moving in and out, it still works. Note: Make sure to hit every long note or you will not be awarded the achievement.

Because it does not specify the difficulty, just play the song on Easy and make sure you hold the whammy bar down the entire song.

Tape down your whammy bar to the guitar controller and play "Number Of The Beast" under any difficulty setting. If you hit every held note you should be awarded the "Whammy Mania" achievement.