Summarize:

This will be the only attack you need. Since you can't attack the mushroom, spam Thundaga on it! This is in case you run out of MP before the mushroom dies. This is for added damage to thunder-based magic. This is for added magic boost. This keyblade is even stronger than Ultima Weapon in terms of magic damage.
Place Thundaga on your shortcut menu. Place some ethers on your shortcut menu. Equip Photon Debugger. Equip Fatal Crest (Final form).