The biggest and baddest bosses yet come your way in the final stretch of Samus' second first person adventure. Be prepared for long fights againts baddies with no understanding of the concept of mercy. Samus will soon get her most powerful weapon in the form of the Annihilator Beam, and just in time to face the ruler of the Ing. Fight on! Just a few more battles to go!
Like the Spider Ball Guardian, the Power Bomb Guardian can't be hurt by your weapons. Instead, you'll have to climb the magnetic tracks leading from the floor to the ceiling, where you'll find four morph ball bomb slots. Bomb each slot to take out a quarter of the boss' health. While it may seem simple, you'll have to contend with attacking Inglets, the corrosive atmosphere of Dark Aether, and the explosive power bombs launched by the boss itself. You should have enough energy by now that this shouldn't be too much of a problem, so just climb to a power bomb slot, bomb it, and repeat three more times.
This time around, Dark Samus will use a wider variety of attacks to try to take you down. She'll charge up before launching herself in a boost ball-like attack, and will bounce around a bit before hitting you with one of her other attacks. Her projectile attacks include a charged blast, a beam that she sweeps around in an arc, a scattering burst, and a barrage of missiles. Dark Samus has clearly come prepared this time around!
Either the power beam or the dark beam will do the job of taking out her health bit by bit, and if you have the Darkburst weapon, use that whenever you can get a clear shot.
As the fight goes on, Dark Samus will disappear, prompting you to switch to your dark visor. At half health, she'll become visible to your combat visor again, and will use the charging attack that she used during your first encounter with her. The closer she gets to defeat, the more often Dark Samus will disappear and reappear, so keep switching visors to finish her off.
Once again you'll be faced with a boss chamber without any safe zones. Time is of the essence in this three part boss battle. First off, 
Quadraxis will fire porjectiles at you, spin itself around like a damaging top, leap around to cause shockwaves, and use lasers to lock on to you to launch a deadly beam attack. The boost ball can be used to avoid the beam, and everything else can be avoided by jumping or strafing away.
Your targets in the first phase of the fight are glowing red spots on the base of the boss' feet. Boost into all four to make your mechanical foe's knee joints vulnerable to super missile blasts. When the blasted leg gives out underneath Quadraxis, you'll know you've dealt some real damage. Repeat for the other three legs to move on to the second phase.
The head will separate from the body, at which time you can switch to the echo visor and fire super missles at the part emitting sonic waves. Doing so will lower the barrier around the head, allowing you to hit the antennae on the head. There are four antennae in all, and you'll probably have to destroy the emitter on the body before going after one of the antennae. Each time the shield goes down, the head will spawn a drone, so don't forget to take those out for energy and ammunition replenishment.
Finally, with all four antennae taken out, the head will hover about the room in a circle, passing by each of the fallen legs. Avoid the missles, shots, and lock-on beams, and fire light beam shots to stun the head. While stunned, the head will move much more slowly, allowing you to climb the legs with the spider ball, and boost onto the magnetically charged head. While on the head, roll into one of the two morph ball bomb slots, and lay a bomb there. You'll drop from the head as the bomb detonates. Do this one more time to finish off Quadraxis.