I always get to level 100 at disc 2... Its easy and dosent take all that time...
When you enter Galbadia garden at the end of disc 2, go to the ice court and fight against the hockey players, they come randomly in numbers.
Maybe many of you know this, but it is good to know.
Most people kill monsters in the Island closest to Hell (has only level 100 monsters) with Quistis' degenerator in disk 3 if they level up at all. It isn't to your advantage to level without bonus abilities. I do find the hockey players funny though.