It has several phazes it goes through during the fight. It has spinning orbs around it that when the player fires on it, the orb hit determines its next attack move. After going through that move, it goes back to the spinning orbs and the player has to hit an orb again. (it can be the same orb over and over if you desire). This continues until either the boss or the player bites it. The player can choose which orb to hit is they have good aim or wait for the spinning to slow down. This can be used as a strategy as some attack modes can give power ups or nukes. The boss is only vulnerable when in an attack pattern.
After the boss blows, the player goes up in a chopper. Unless playing the game on a lower difficulty, the boss gets grey armor with 4 tentacle arms that follows the player up the way out where they fight to the end. The boss attacks from below with tackles and tentacle strikes. The player has to jump when needed and aim down and blast. Oddly enough, its armored form takes far less hits than just the undefended brain.