How do you get the ultima weapon? Plus tips!

How do we get it? Like can you get it near the end of the game or you can get it whenever, what can you do to get it?


Plus, I have played Kingdom Hearts 2 for awhile and like I am not really good at the fights, like I have my cousin do it for me. Like is there any tips? Can we like train or something like that? What helps you be a pro at this game? Cause I wanna be one. Like its also about leveling up and stuff too and how do you do that? Give me ALOT of tips, I wanna play better then my cousin! D:
He is like 6 years old! xD