Whether or not you claim the Fierce Deity’s Mask, speaking with the lonely youth who wears Majora’s Mask triggers the final battle. Prepare yourself for an epic clash.
Phase One: Majora’s Mask
During the first phase of this three-stage battle, the four guardian masks fly out from Link’s grasp and become stuck to the walls. Majora’s Mask then emerges from the far wall, sprouts tentacles, and begins the attack.
Majora’s Mask can’t be harmed while it’s facing you, so don’t bother attacking it. You must stun the demon first instead. You can do this by shooting an arrow into the back of the mask, but this can be tricky. To make things easier, lock on and hold 
to ready a spin attack. Unleash the spin attack when Majora’s Mask spins and flies down at you. (You can also simply strike Majora’s Mask with a normal sword attack as it draws near.)
Once you’ve countered Majora’s Mask’s spin attack, the mask is briefly stunned. Race over, and strike it once with Link’s blade. Repeat this until the four spirit guardian masks pop out of the walls and join the fight.
The other masks have attacks of their own, making the fight quite chaotic. Lock on and shoot arrows at the other masks to defeat them (it takes three regular arrows to defeat each mask), or simply strike them with Link’s sword whenever they’re in range (or use the Hookshot to pull them close). Once the other masks are gone, you can focus on battling Majora’s Mask.
Majora’s Mask also starts firing a punishing laser once the other masks join the battle, but Link’s Mirror Shield can block this laser and even reflect it back at Majora’s Mask. Lock on, and back away from Majora’s Mask while holding 
to defend. Keep backing up until you reflect the beam back at the boss.
Continue reflecting Majora’s Mask’s laser, and then stun it by intercepting it with a spin attack (or by shooting it with arrows from behind). Inflict damage with Link’s blade each time the mask drops to the floor, and keep it up until you defeat it.
Phase Two: Majora’s Incarnation
After suffering significant damage, Majora’s Mask sprouts arms and legs, transforming into Majora’s Incarnation. The battle changes at this point, and so must your tactics.
Majora’s Incarnation sprints around in a mad rage, performing a number of disturbing dances. Sometimes, the fiend stops to fire a barrage of painful projectiles at you. Lock on with 
-Targeting, and keep your shield up to block these projectiles as you chase after the boss.
Try to strike Majora’s Incarnation’s legs with your sword as it skitters past. Holding 
to charge up great spins is a great way to do this. Any successful sword strike trips Majora’s Incarnation and causes it to fall to the ground, stunned.
Now’s your chance! Race over to Majora’s Incarnation, and slash it with Link’s sword. You can land one or two hits before the fiend recovers and resumes its mad dance. Keep tripping Majora’s Incarnation and scoring damage with Link’s blade until you advance to the battle’s third and final phase.
Phase Three: Majora’s Wrath
Majora’s final form, Majora’s Wrath, is the most powerful of all. Don’t give up now, hero—you’ve almost won!
Majora’s Wrath has long tentacles for arms, and it uses these to lash out at you with surprising range and speed. Keep your shield up to block these swift, far-reaching strikes, and try to stun Majora’s Wrath with a long-range Light Arrow.
Once you’ve stunned Majora’s Wrath with a Light Arrow, rush toward the boss and unleash the full fury of Link’s sword. You can land two hits before the brute recovers.
As the battle wears on, Majora’s Wrath adds new and more punishing attacks. It wraps you up with its tentacles and tosses you across the room—an attack that can’t be blocked. It also sends out spike-covered spinners that pursue you around the arena. Block these spinners to bounce them back as you continue to stun their master with Light Arrows.
Even if you run out of magic power, you can still stun Majora’s Wrath with normal Arrows. You can find more arrows by breaking the jars around the arena, so don’t worry about running out. Just keep firing Light Arrows to stun the Majora’s Wrath, then close in and strike hard with Link’s blade. Don’t relent until the demon is at last undone.
Termina is Saved!
Congratulations, hero, you’ve just beaten the game! This guide won’t spoil the ending for you, but hopefully, you enjoy it. Save your game after the credits roll, and the next time you play, you’ll find yourself back in Clock Town on the Dawn of the First Day, with all of your masks back in your pack—including the Fierce Deity’s Mask.