In the room before Dark Samus on planet Phaaze, there is a small Leviathan seed in the middle. Pull off the two sides of it using your Grapple Beam first. Then while dodging and blowing up Metroids, shoot the Leviathan Infant inside with single shots from your Hyper Beam. It will continually switch sides. Follow it. It should be dead after about five minutes of this.