Ok mvgt the thing about that big cataur.
1. It's a GF
2. yes it's very hard to beat
3. no it's definately not invicable
4. once you've suceeded in beating it once, guess what DON'T think that you've won and that the battle over and that you have a GF, oh no you have to face it again, coz it runs away after you've first beat it and then if you want the Gf you'll have to find it again, once you have beaten it the first time guess what, you have to beat it 6 more times! then and only then can you relax and say that you beat it and also that you have another GF. is that ok for you?

I'm not to sure on the fish bone thing, sorry, coz in the last stage you have to fight monsters to get your abilities back right, well I got as far as that, I got some of my abilities back (you only really need, the ability to: have Item, Draw, Magic, Limit breaks, save I'm sure theres one more, can't think of it) any how once I did that I didn't have a clue as what to do, as at this point I didn't have my Guid book to help me get through the castle, so I asked my kind brother to get me to her main room and he did and I didn't watch so I'm affraid I can't help you with the fishbone thing, sorry.