The leftmost part of the monkey bars represents a safe spot from the B.O.X's jump attacks. In the second fight, the rightmost part works too, and the B.O.X.'s missiles can be dispatched easily with the Plasma Beam. A screenshot of Samus battling the B.O.X.

Stick to the low ground and fire charged beam shots directly at the SA-X to destroy it. Your beam must be fully charged to damage it. As it approaches, Space Jump over the SA-X to the other side of the floor while charging your beam, and release the shot when you land.

Stay toward the left side of the room to protect yourself from its jumping attacks, and fire off several missiles diagonally to the sweet spot both before and after it attacks with missiles of its own. Destroy the oncoming missiles with your blaster, and then get yourself into position for your next attack.

Thankfully, this boss creature is not nearly as fearsome as it looks. It begins to attack Samus by jumping high into the air and across the room. Simply dodge these attacks and wait for it to float down to the ground after the third or fourth jump.