I believe that Ultimecia and Necron in VIII can still give you a challenge I remember losing frequently to necron when I first started IX, well I was 10 then so that's probably why, but I don't think Ozma's that hard either because I had beat him on my second try. With XII I agree he can be far too easy I beat him on my second try on my first playthrough, and now I'm trying to complete a lot of sidequests so he'll probably be very easy to beat, most of the later hunts are harder than him. Maybe someone should implement some kind of way to increase the level of the final boss as you get stronger if you so choose or a third option that the final boss will be the toughest when you reach Lvl 99.