Only the second stage boss and onwards have a form change. You can tell when a boss will change forms by a small horizontal line which splits the bosses health bar into segments. Once enough damage is dealt so that the bosses Health goes below that point, they will change form. I suggest doing this on stage one. Simply get to the stage one boss and conserve your phantom energy. Damage the stage one boss until he is about to die, then enter phantom form with and shoot him to finish him off to unlock the achievement.