1. Beast's Castle's Mushroom is a win or lose situation. You either get enough or you don't. The best way is to have Glide maxed and Dodge roll maxed, and use those two abilities efficiently in trying to gather the orbs he drops.
2. Final Form + Thunder = the way to go. Have at least 3 or 4 eithers with you, also. As soon as you run out of magic power, just use an either and continue the thunder spree.
3. I found, for this one, the best way is to use berserk. Make sure your MP hastes are on, equip Fenrir, put on horizontal slash, and cast cure on yourself right before starting the fight. Pull him out, attack him mid-air once with O, then start whoring the Side Slashes. Once you get toward the end and you get your magic back (be watchful of your MP! xD) use trinity limit for Sora. That can win you the last 20-25 hits.
4. Well, I found the easiest was to take off all combo +'s, have Fenrir equiped, and just keep firing your end-combo magic shots. Also, memorizing the patterns of this Mushroom helps tremendously.
