The infamous ExDeath trick...

I tried a minor version of it to level some of the villains but in the end, I decided not to. It doesn't help leveling skills and several of your characters early skills are awesome once you master them and get the branching attack with it. My other issue with quick leveling is that it doesn't help you with learning the characters and I find from time to time that its harder to use high level abilities with a character if you didn't learn the basics.

Its a great trick if you have no plans on using the character (I may need it for Tidus) but I feel its better to grow with the character if you intend on using them as a main.