Short time after you received the purification power you will face Ixtab, the boss wearing the last mask sealing the door to the well of darkness.
This time you are facing a three-phase boss fight:
After entering the arena and the usual chatter you will notice that Ixtab is standing in a huge puddle of darkness. Press RT+X and let Majin Use his newly achieved power on the puddle. This results in Ixtab being stuck on the ground. Go to the back end of her spider-like body and hit the ugly red-glowing bubbles. Repeat a couple of times.
Don't stand in one place! Keep moving and try to catch her attention or otherwise her attacks will interrupt Majins purification power!
After you literally kicked her a**, she'll move to the next area. Heal yourself and Majin if necessary and then follow her.
Ixtab is standing in another puddle of darkness fed by three small wells. Press RT&X again and let Majin destroy all three of them. Ixtab will try prevent Majin to destroy her wells of power. There's not much you can do about unless trying to catch her attention.
After Majin has destroyed all wells, brace yourself for phase 3.
Is very similar to phase1. Keep moving and prevent yourself from getting hit. Press RT&X to let Majin freeze the puddle of darkness. When Ixtab is stuck, hit the mask on her face (don't concentrate on the red life ball just yet). Repeat until the red ball pops out of her head, then press B to kill the beast! Done!
Make sure to have an eye on Majin's health bar at all times. If he's running low on energy in the second area, just make him follow you behind the stone walls and give him a healing fruit!