I actually found this to be quite a difficult final boss. The first time I played Majora's Mask, I didn't even know about the Gilded Sword sidequest; I didn't even know about the Great Fairy's Sword; I didn't even know about the Fierce Deity's Mask. I walked into this battle with the Kokiri Sword, and only half magic because I didn't get the upgrade. This actually caused Majora to be quite a significant challenge, because attacks did very little damage, and I actually had to get dangerously close to hit him because the sword had such little range. His close range attacks, combined with his long range attacks, not to mention the demonic masks on the wall, caused this boss fight to provide a significant challenge to me.
What I'm trying to say is basically the same thing that everybody above me said. The difficulty of this boss battle is all up to the player, and the way he/she wants to fight it. If you want to upgrade your sword, be my guest. But keep in mind that it makes the fight slightly easier. Want to get the Great Fairy's Sword? Great! Not that you're forced to use it, unlike the Gilded Sword. And same goes for the Fierce Deity's Mask. Only use it if you want the battle to be a piece of cake that's over in 5 minutes. So basically, the point is, the battle is only as easy as the player makes it out to be. If you have all these upgrades, the only one you have to use is the Gilded Sword, which is actually the hardest of the options. So, overall, this is one of the tougher bosses in the series, but can be made easier if you use the proper resources that are available.
What I'm trying to say is basically the same thing that everybody above me said. The difficulty of this boss battle is all up to the player, and the way he/she wants to fight it. If you want to upgrade your sword, be my guest. But keep in mind that it makes the fight slightly easier. Want to get the Great Fairy's Sword? Great! Not that you're forced to use it, unlike the Gilded Sword. And same goes for the Fierce Deity's Mask. Only use it if you want the battle to be a piece of cake that's over in 5 minutes. So basically, the point is, the battle is only as easy as the player makes it out to be. If you have all these upgrades, the only one you have to use is the Gilded Sword, which is actually the hardest of the options. So, overall, this is one of the tougher bosses in the series, but can be made easier if you use the proper resources that are available.