Goht

Goht is probably the easiest boss in Zelda: Majora's Mask.  To defeat him, Link must first shoot a fire arrow at the icy block that is holding him, and then put on the Goron Mask.  Goron Link must then roll up into a ball and attack Goht with the spikes that come out of him.  Since this consumes magic, Link must aim for the green jars that are scattered throughout the room to get magic refills.  If Goht gets too far ahead of Link, he will shoot lightning at him, so try to avoid it.  When Goht is almost defeated, he will begin to throw bombs back at Link, so dodge them, and keep hitting Goht with your spikes.

Gyorg

Gyorg is one of the toughest bosses to beat if you don't know the proper strategy.  The best way to defeat him is a combination of using arrows and one of Zora Link's special abilities.  When Gyrog is in the water, Link should shoot an arrow into his eye to stun him (Z-targeting works the best).  He should then turn into Zora Link and dive down into the water and use the barrier attack (press the R button) to hit him in the face.  Immediately swim away from Gyrog and back onto the platform in the middle of the room and repeat this.  About halfway through the battle, Gyrog spits out several small fish.  If they get in your way, use a barrier attack on them.  They leave behind hearts that will restore your energy.  These fish will hurt you if they touch you, so be careful if you decide not to attack them. 

Twin-Mold

When you look at this boss, one thing comes to mind: Lanmolas, the sand monsters from the past Zelda games.  As the name suggests, there are actually two monsters to defeat.  The best way to defeat Twin-Mold is to use the Giant's Mask, but the drawback is that the mask uses up one magic point for every second you wear it, so you will need some green potions if you want to use it the entire battle.  Defeating Twin-Mold is straightforward.  Just hit him with your sword or arrows in the tail.  It helps to use the Great Fairy's sword, so it's a very good idea to find the fifteen stray fairies in this dungeon and go get the sword before fighting Twin-Mold.  Not only does this sword do four times the damage of the Kokiri Sword, but it also has a longer blade.  If Link runs out of Magic, Arrows are the best way to hit Twin-Mold, although it is still possible to hit him with your sword.  You should wear the bunny hood to run faster.  If you run out of arrows, just try to guess where Twin-Mold will land and be ready to hit that tail.  

Odolwa

Odolwa looks a lot harder than he actually is.  The battle itself is actually very straightforward.  Link just needs to get close enough to hit him with his sword.  The main trick is avoiding Odolwa's attacks.  It is easy enough to just avoid or use your shield on his sword, but he also can summon smaller enemies to attack you.  Just defeat the small enemies and collect the items (usually hearts) they leave behind.  If he surrounds you with a ring of fire, there is nothing you can do.  Just wait for the fire to go out and concentrate on hitting him with your sword.  After a few hits, he will be defeated. 

Skull Kid 

The only time Link actually has to fight Skull Kid is at the end of the third day before he has the Ocarina of Time.  Link will still be stuck in Deku Link form, so he should shoot a bubble at Skull Kid.  After hitting him once, Skull Kid will drop the Ocarina of Time and Link can warp back to the beginning of the first day.

You will face Skull Kid again at the very end of the game, but don't fight him.  Instead, play the Oath to Order and it will take you to the area where you fight Majora's Mask.

Final Boss: Majora's Mask

Majora's Mask looks like the easiest enemy in the game, but in fact, it is the hardest.  The mask has a wide variety of attacks, and has an incredible defense.  But if you transform into Oni-Link then you have nothing to worry about.  He actually has three forms.  The first one is just the mask with some tentacles coming out his back side.  Link should Z-target him and when he tries to dive-bomb link, slash his backside with your sword.  It takes several tries to pull off, but just be patient and you will hit him.  After a successful hit, Majora's Mask will fall down in a corner of the room for a second.  Quickly run up to it and hit it with your sword again.  After a few hits, the remains masks will come off the wall and attack you.  Be careful of Majora's Mask's fire attack, but shoot each remains mask with three arrows.  Then concentrate on Majora's Mask again.  This time he will shoot fire at you.  Use the mirror shield to reflect the fire right back at Majora's Mask.  After a few hits, the mask transforms into it's second form.

In its second form, Majora's Mask grows long legs and runs quickly around the room, stopping only to shoot energy beams at you.  These beams can be blocked with your shield for a while, but eventually one will hurt you.  Thankfully, they don't do a lot of damage.  When Majora's Mask gets close to you, hit it with your sword in the legs.  This will knock it down long enough for you to run up to it again and hit it with your sword a few times.  After repeating this, the mask will transform into it's final form.  Z-targeting is very useful in this battle. 

In the mask's final form, it sprouts whip-like arms and can hit you from very far across the room.  Shoot a Light Arrow at the mask to stun it, then quickly run up to it and hit it with your sword as many times as you can.  It is tough to dodge those whip like arms since they can hit you from almost anywhere, but if you stay back he probably won't hit you.  If you run out of magic or arrows, there are a few jars around the wall you can break to get more.  Repeat these steps and Majora's Mask will be defeated!