Core-X are the animating force behind the bosses of Metroid Fusion. After the boss form has beet sufficiently damaged, they discard the body and engage Samus in their true form. Once the X has been broken open, Samus gains whatever power was exhibited by the boss, except in rare cases, such as the the Core-X inside of Ridley, which grants the screw attack, or several of the Beam Core-X.
Standard Core-X can engage only by swooping towards Samus, but the specialized Beam Core-X can attack by firing a beam weapon at Samus when they open up their sole eye. The Beam Core-X are made even more difficult because they are only vulnerable in the eye, and even then it must be open.
Core-X are vulnerable only to missiles
, but will release standard X when shot with a beam. This makes replenishing lost energy and missiles much easier. As the game progresses, they become progressively more difficult to destroy.