I'm at the hyrule castle and I am battling the beast form of ganondorf..

Well you are a lucky guy because I alraedy beat this game. Ok when he is in the beast form he will disapear well when he appears again you will shoot arrows on his head. Once you keep doing that he will eventually dodge your arrows, thats when that link wolf springs to action. remember in the beggining you had stop the animals whenever you talked to the sumo man well do the same to ganon. Eventually you will beat him

An easier way to beat beast ganon is to be in wolf form the whole time and watch the holes to see which one he is coming out of. Then Midna's hair will becoming a hand and you will hold A while moving the control stick back and forth then you can jump on him and bite his belly a or turn into a human and do a great spin which will hit him twice in one swing.

When he disappears look for a big shadow under you or portals. If there's a big shadow under you run! If you see portals take out your bow and watch them when one turns blue expect Gannon to come out, then shoot you arrow run to his belly and slash it until he gets up again, repeat this until the arrows stop working. (He'll disappear when you shoot it.) When they do turn into a wolf and when he comes out of a portal it will say "A grab" press and hold A and turn the control stick left or right and Midna will throw him over, turn into a human and slash him.

I can't remeber exactly but you wait for him to jump into a sort of teleportation wall. Wait for him to come out. Use the grab thing with the iron boots (like the ram at the start of the game). Then when he is down. I think you turn into the wolf and attack his belly. Check other reliable sources.