In the second part of the fight Malefors weakness is the crystal plate strapped to his torso-when it glows purple you can attack it with your breath attacks. You may notice it takes FOREVER for it to start glowing-but with this method you can beat him quick.
Firts he will line up some small crystals to fire at you-smash all the GLOWING crystals with Cynders beam attack BEFORE HE FIRES THEM. Then the crystal plate will glow, (that was quick!) so attack it until Malefor falls.
He will rise up again and start shooting random crystals at you. Dodge his attacks then he will line up another batch of crystals-smash them, and look, the plate's glowing again! Another chance to attack. Repeat the process, being mindful that Malefors attacks will gradually become faster and more frequent. After five rounds, Malefor is finished....and so is the game!
All that in much less than five minutes.... if you don't smash the crystals it would take about ten, and your chances of dying would he higher.