It depends on which form

1rst form shoot his eyes and when u do that something will happen so then hit him with ur sword

2nd formdo the same things

2rd ur almost done just make all kinds of forms of u and attack his eyes and then hit his legs with the staff beam then shrink and go inside his leg and destor it do the same with all his legs

then ur done u go though the credits and they r really long
if u have any questoins just ask me