thanks.the replies really give me some tips,i will try it again.
and u 2 remind me something.when i was in the battle,i used some magic that can make the enemy become asleep and lose its ability to attack.then i attack it fiercely with limit breaks and all kinds of magics,especially those of water.whenever it waked up,i would try to do the same thing again.yes,it did run away several times,maybe about 3 or 4 times.but after that,it became a one that would never escape!i have to fought it until its HP ran out.but its HP is incredible,as i said before,i played with it for about 1 hour,using all approaches i can imagine,consuming neally all items that could do harm to it,and the screen displayed clearly that it lost its HP sharply,but gust what?all that damage i did just seemed to be a piece of cake to it! no others could withstand such kinds of attack,neigther could the Omega Weapon,which i got really tired to fought it but finally i was able to kick its .

and about the fishbone....sorry,i m not quite sure whether it was really a fishbone or a key.but it's something shining that lay on a wooden hanging bridge over a river.when my team walked on,the bridge waved,and that thing was waved off down the bridge into the darkness below.it was so fun that i went to find it.then i went to the piano,finding that whenever i pressed a key,one of the steel bars of the gate that keep me away from reaching the river went down.so i pounded down all the keys on the piano at the same time,and the gate opened,so i was able to reach the river that the fishbone(or key or sth. shining) might have fallen into.well,i really saw that thing lying on the river bank!but i cannot pick it up cause something like a stone or so(i can't remember clearly) stood in my way.i just wondering how to get the key and what is it used for.