Link will begin to radiate with a bright light and the Guardian Masks which he spent so much time and effort to get his hands on will fly over to take their places on the walls. Then the evilest mask of them all (no, it's not the Bunny Hood) will awake and the battle will begin.
This is one mean mask, it's huge (by mask standards) and it's got about a million red slimy tentacles, these are what you're aiming for. Majora's Mask will spin through the air and then it'll swoop down on you, block it with your shield and then when it's underside is your line of sight, shoot it an arrow. When it falls run in and chop it, you should be able to get in two jump slashes before it flys back up. After two jump slashes, you'll see a short cinema of the Guardian Masks coming to life and joining Majora's Mask in hovering around the battle room. The Guardian Masks will fire off energy balls at you, while Majora's Mask will target you with a scorching fire beam. Fortunately though, you can reflect this with your Mirror Shield. Don't lock on to Majora's Mask, or any of the other masks, just make sure the beam will hit your shield, and then aim it at any of the Guardian Masks or straight back at Majora's Mask. The Guardian Masks will burn up if you leave the beam on them for a second or two, but three arrows to each will also make them fall. Majora's Mask, is not so easy though, you can either burn it until it falls or repeat the process of stunning it with arrows then running in to do two jump slashes. Once you've damaged it enough you'll view a cinema of Majora's Mask bursting into flames. So it's all over then? Not even close!