The SA-X (named by Adam and presumably short for Samus Aran-X) was the main antagonist of Metroid Fusion and the eleventh boss Samus faces on the Biologic Space Laboratories research station. It is an X Parasite mimicking Samus Aran and her Power Suit.

So, go into Morph Ball form and stay near the corners, moving accordingly to avoid him. Occasionally he'll open his mouth to spit fireballs at you; quickly get out of Morph Ball form, run under him and shoot a few missiles inside his mouth to damage him. As you hurt him he'll gradually turn red.

Your best strategy against the Omega Metroid is to attack it as the SA-X did: fire charged Ice Beam shots directly into its belly. Charge your beam and run at the creature, and then hold the L button and release the shot directly into the Omega Metroid's chest.