The King Worm can be fought multiple times, as it runs away if you don't kill quickly enough. After it blocks you in, the King Worm's head will appear at the top-middle of the screen. When it opens its eye, immediately shoot it with a Missile. It will then move straight downward, so move to the left or right of it to avoid getting hit. Three hits with a Missile will kill it. Beware touching its spiky body on either side of the screen, but be sure to shoot the spikes that fall off it for extra Health and Missile ammo.