you have to go overboard at the begginning to attack Clayton. then you still just attack the lizard like 50 to 80 times. once Clayton falls off use lightning until he dies. equip dodge roll and stock...
You don't need a certain level to beat him, but the only thing I suggest is you bring enough hi-potions and give some to goofy (big inventory) he'll heal you when it's needed. try attacking him from...
You can play another game or play the holo missions on the game 2 unlock more items. I believe you can unlock Sora when you do that. You can also play the game again, or watch the movies in theater...
Sorry no!
Once you complete the game the ending scene happens. And then credits. So i suggest you do everything you wanna do before you complete it. The ending scene starts a teaser of kingdom...