To get the Fierce Diety mask, get all 20 happy masks(not including transformal masks), then beat all the temples. When all the guardians return to normal, use the song of time. You must then beat all the bosses all over again WITHOUT using the song of time. After that, go to clock tower at midnight on day 3. When on the battle with skull kid, play the oath to order. The 4 guardians will stop the moon from falling. Then, you can get on the moon. Go to the tree to see kids in masks. Give all your masks to the children (don't go to the kid in majora's mask yet). After you gave away all your masks, go to the kid in majora's mask, and he will give you the fierce diety mask.