Thanks everyone for all your help. I actually finished the game last night as i'm planning on starting it again- I want to get 100% this time and the extra endings. I actually found the encounters with Vegnagun and Shuyin pretty easy, which was surprising- I didn't even need to use catnip! I was a little dissappointed with the ending (I think it was the normal ending I saw) to be honest, I guess I was expecting Tidus to reappear. Though I think I read on another post that he does make an appearance in one of the other endings.
And no, I couldn't beat Chac which is partly why I decided to start again.