Acid Worm

Its only weak spot is its eyes, which can take tons of damage once being shot by your missiles. The rail system above the boss will allow Samus to easily cross to the other side. She'll have to use this to her advantage in order to make the worm crash into the blocks that are scattered throughout the stage, thus allowing her to easily shoot its eyes.

Later on during the fight, the colossal worm will start to move faster, requiring faster reflexes from Samus.