Instructions
Play through the game until you get to the Quarantine Cave in Phendrana Drifts after you have gotten the Thermal Visor from the Research Lab. Once you have the visor, backtrack to the save station on Phendrana Drifts and save your game.
Exit the Save Station and move directly forward until you reach the huge doorway with the mechanical arch over it. Blast the center of the arch to open the door and proceed through the cavern. Shoot the door to open it, then use the Morph Ball track to get through this area.
Shoot the door at the end of the Morph Ball track and walk through the cavern to reach a large open area with a massive pile of rocks. Jump down to the pile of rocks to initiate a scene wherein the rocks transform into Thardus and a boss fight ensues.
Equip your Thermal Visor as soon as you regain control of Samus. This reveals Thardus' weak points, but they only appear one at a time. The first is his right kneecap. Remove the Thermal Visor and shoot the weak point until it breaks.
Avoid Thardus' attacks. He shoots large rocks at you, which you can shoot to destroy. He also morphs into a huge ball, so go into Morph Ball mode and use your Morph Ball Boost to get away from him. You can also use your Morph Ball bombs to attack him while he's in this state. Keep using the Thermal Visor to reveal his weak points and shoot him until he is completely destroyed. When the battle is over, you are rewarded with the Spider Ball.