many types, one continent, one person.
I'm going to guess it is a monster that is humanoid in form... I don't know which one would work... The second line makes me think the answer is around Midgar.
many beasts, one area, forever.
A monster that you can fight in this one location - for the entirety of the game {forever}.
satan's angel on the ground
This strikes my as Midgar Zolom (Spelling?).
