Synthesis materials a must if you want the Ultima Weapon or any of several extremely useful items. If you are having a hard time collecting enough materials for synthesis, try to put both Donald and Goofy into your party, then equip Sora, Donald, and Goofy with the "Lucky Lucky" ability (5 AP to equip, Sora won't acquire it until lvl.99 if you chose the staff with Roxas after meeting Namine).

Equip them with the weapons that have the "Lucky Lucky" ability: Sora- Sweet Memories keyblade (100-Acre Wood), Donald - Meteor Staff, Goofy - Genji Shield (Olympus Colussium). Finally, equip one of the three with the Lucky Ring accessory for a total of 7 Lucky-Luckys. Enemies should drop items at a far greater rate, sometimes even two items at once.