Let's start with the kid wearing the Odolwa mask. He'll ask for one mask. After you hand over a useless mask, play hide and seek. Become deku Link and glide from flower to flower. I hope you've been practicing! The exit is in the far left corner of the room. Not too difficult, I hope. Watch out for the spinning dealies in the center of the room and make sure your timing is true. Talk to the kid when you find him in the next room and give him one more mask. After talking about friends, you'll be brought back to the tree.
Next, the hardest one. The Goht kid. After forking over TWO MASKS, he'll play hide and seek. Goron Link is the way to go. Line up on that straight away, dead center and roll. Once you have the spikes out, release the joystick. Just let him roll. He'll ricochet off the chests and fly off the ramps. Once you see a line of magic pots, stop and walk over the wooden plank. See the platform with the ramps leading up to it to your left? Just let goron Link go and avoid the swirling portal. It takes you back to beginning. Once on the platform, go through the door and talk to the kid, giving up two more masks and getting more confused about friends.
Gyorg kid wants three masks. Jeez, they are getting greedy. This trial is easy. Hop into the water as zora Link and take the right path, then left, left again and finally a right. Talk to the kid, give three more masks ( >_< ) and warp back to the tree. One more trial to go.
Twinmold kid. The final kid. Wants... four... freaking... masks... FOUR! >_O *twitch* There are only four left! Anyways, this trial just measures your swordie skills. This one is also relatively easy. Just make your way through the trials and you'll be a happy camper. First, you'll be up against a dinofols. Then defeat the garo master once you enter the next room. The next room has an iron knuckle in it so beat him up some. Shoot a bombchu at the crack in the wall by letting it go and shooting it with an arrow to explode it as it's riding over the crack. Activate the switch and get to the next room. Do the same bombchu trick as you did before, only this time it's on the ceiling. Fire arrow the next switch and walk your way through that door. The kid will talk more about stuff that is weird and teleport you to the tree after taking your last four masks.
Now you may talk to Majora. If you have given all 20 masks to the kids, he will give you the last mask. The Fierce Deity mask! And enter you into the final fight for Termina... you must inevitably fight...
Pretty wussy for a final boss... or is he (she?)? Yes, he/she is. (No gender assumption here!) If you are using the Fierce Deity's mask, then shame on you. Just z-target and press B until it is defeated. If you're not a CHEATER, then the vunerable part of Majora's mask is the back, where all the tenticles are hanging out. First, wait until you have a clear shot of the back and hit it with an arrow (best to do this when it is spinning around the room since it won't be able to turn and block the arrow.) You can also use zora Link's flying fins if you run out of arrows. Once you peg it, it will fall to the ground and you get a few free smacks at it with your sword. After a few hits, Majora's mask summons its mask friends. Arrow or zora fin them to get them out of the way and concentrate on Majora's mask again. It will have a new ability, a fiery laser so reflect it back with your shield. To hit Majora's mask, it is better to aim the shield while NOT z-targeted.
Majora's mask attacks are easy to avoid. When it is floating in the air, spinning around like a frisbee of death, arrow it or sidejump to saftey. When it summons the boss masks, kill them. It'll eventually shoot lasers at you which are reflectable by your mirror shield. Keep using this strategy and it will die... or will it?
A final boss with a second, presumably more powerful form? How original... But alas, this one is less powerful than the original mask. Though it now has a body, its body is weak. Weak like a baby! A baby with ridiculous running speed and the ability to shoot shining, hurty lights. If you are still using the Fierce Deity's mask, z-target and press B. Really, though... is there any other strategy if you are using that mask... The REAL strategy to fighting this guy is simple, hit it with arrows, fins or your sword to trip him up and smack him when it's down. It'll stand up, hold its head and stay still for a bit for some reason, so sword its legs again to make him fall on his face. Lather, rinse, repeat if desired and you SHOULD desire since it will make you win! ^_^ You can also do what Tatl suggests and throw your buff, spiky goron body at it, but it's too small a room to do that, I think.
Its attacks are terrible... It'll run around the room and run over you... Sometimes, it stops and shoots white lights at you which you can sidejump away from. That is all... Smack it for being weak and stupid and it will die... or will it?
Sooooo... Majora's Incarnation was hooked up to a bicycle pump and now it has WRATH!!!! RRRAAAWWWRRR! This is the EASIEST of the three forms! >_< For the Fierce Deity's mask users, z-target and press B (cheaters ;_; ...you make baby Mario cry.) For those manly men and (insert feminine equivalent to manly here) women, put up your shield, z-target if you want and walk on over to Majora's Wrath. Slash, slash, stab and stab.
Majora's Wrath has very fancy whip attacks. These can easily be blocked with your shield. After the first few hits, Majora's Wrath become more evasive, so you will have to be quick when approaching. It may also kick you or use a rapid whipping attack. After a few more hits (say six or so) it will unleach a new attack. Two tentacles... what can tentacles do...? What do tentacles always do...? They grab you and throw you. It is hard to predict this attack and blocking with your shield does nothing so take it like the Hero of Time! The last attack is does is not really an attack... More like summons giant Beyblades (I'm not kidding, sadly...) and directs them with its tentacles. After a few more smacks with your sword and it will die... or will it? ...Yes, yes it will. Finally. And now, my friends... Termina is safe. ^_^ Everyone is happy and alive, except the deku butler's son, Darmani and Mikau. 99% of the population saved isn't bad, I guess.