How do you beat skull kid and how do you get fierce dietys mask?

To beat skull kid I'm guessing you have completed all temples .Play the oath to order on your orcarina then there will be a cut scene for a moment I wont tell what happens. The answer to your second question is you have to get all masks then when you get sucked into the moon you will be in a field with a tree in the middlebut if you look at the childrens faces there wearing masks of the boss's that you destroyed at the temples and one with majoras mask.If you want the fierce dyties mask DONT TALK TO THE CHILD WITH THE MAJORAS MASK YET.Talk to the other ones and they will want to play hide and seek with you if you find one he will ask you for a mask and more until he has enough do this wiht all four you get the firce dytie mask then talk the majora kid and you will fight majora.You don't need firce dytie to defeat majora.