Yup, there are two XP tricks. I only know one.

You have to equip the following abilities to your weapons (of three persons):
Triple AP (double AP will also do)
Triple Overdrive (double overdrive will also do)
Overdrive -> AP (important!)

Then, fight Don Tonberry. One of the three characters will be attacking, and because the Tonberry counters everything with a very strong attack, you're attacker will die often. The two other characters are going to heal the attacking character. Importent is the type of Overdrive you have, the attacker's overdrive should be set to 'Stoic', the other two's set to 'comrade'. Every time the attacker is dealt damage, you earn AP. So, even if you flee from the battle, you get tons of AP.

The first try for me earned me like 60 sphere's for the three persons, even more i think.