Edit: The two previous posts were not there when I wrote this one.

"when you cast blind on diablos (the boss) will he always miss on physical attacks?"
He won't ALWAYS miss. He'll usually miss, but he can on occassion actually connect an attack. But you can probably beat him in enough time where that wouldn't be a problem.

"is it true that diablos casts curaga on you when you hit him with demi? becasue it doesnt work for me.. or do i have to do it stocked.. and then casted?"
It is true, but you have to cast it on him from your stock, not while drawing it from him.