i love ff8,but i have some puzzles that i can't solve all by myself,i hope someone can help me:

(in fact,i finished this game about 3 or 4 years ago,so i have to recall my memories hard,and i hope i won't say sth. wrong)

1 i remember i encountered a very huge cactus on an island,and then i started fighting it.but its HP seemed so high that no matter how hard i tried (i fought it with the most powerful and effective attack,costing its HP more than 9999 each round,and the battle lasted for about 1 hour),how long i fought,i ended up with an game over.is this big cactus unbeatable!?

2 in the last stage,there is an hanging bridge,and i found sth. like a fishbone lying on the edge of it.when i got on the bridge,the fishbone fell down.after playing a piano correctly,i was able to open a gate near a river.i passed through the gate,and found the shining fishbone that droped from the bridge lying on the river bank,but i couldn't pick it up.does anyone know how to get that fishbone and what exactly it is used for?