|“||How did that X download the Varia Suit data? This doesn't seem to make any sense at all, unless the X... Unless the X have the ability to process data organically...||„|
The Mega Core-X is a large Core-X with miniature variants of itself surrounding it. Before the battle, it is seen downloading Galactic Federation data of the Varia Suit in a Data Room, thus turning the normal Core-X into the Varia-armored Mega Core-X in the next room. During the battle, its only strategy is attempting to ram into Samus. To defeat it, Samus must use her Charge Beam, as only fully charged shots will damage it. Firing a charged shot at the smaller Core-X orbiting the main body will destroy them, reducing both the number of obstacles protecting the Mega Core-X and the boss's attack range. The Spin Jump Attack can also destroy the small Core-X, and can even be used to damage the Mega Core-X itself (although doing so will result in Samus taking damage as well). Samus's Missiles will not damage the Mega Core-X or any of the smaller X orbiting it; incorporating the Varia Suit data evidently rendered it immune to a Core-X's normal weakness, much like Samus's Varia Suit protects her from extreme temperatures.
It should be noted that while Samus's movements in the Water without the Gravity Suit are incredibly hampered, the Mega Core-X moves at the same rate regardless of the presence of water (or walls, for that matter). If Samus slips off the platforms, she will be at a sore disadvantage until she can get out of the water again.
Once Samus has defeated the Mega Core-X, it will revert to a normal Core-X; it loses its invulnerability to Missiles, but can no longer be harmed with the Charge Beam. Once Samus has absorbed that, she will recover the stolen Varia Suit data.
- As it downloads the Varia Suit, the core's casing can be shot with Super Missiles, though it won't generally affect the boss itself.
- This creature is the only X-based boss in Fusion that did not infect/mimic an organism.
- It shares its boss music and damage sound effects with the Zazabi.
- Humorously, Metroid Fusion: The Official Nintendo Player's Guide describes the Mega Core-X, which it refers to as simply "creature", as resembling a "giant, deadly beach ball."