Go to the Stillshrine of Miriam. Find the Rare Game Negalmuur, and set your gambits like this.
Party Leader
1. Ally: any...................Raise
2. Ally: HP<40%................Curaga
3. Ally: any...................Esuna
4. Self: MP<20%................Charge
5. Foe: HP>/=5,000.............Haste
6. Foe: HP<3000................Attack
7. Ally: any...................Regen
8. Ally: any...................Haste
Second Member
1. Self........................Decoy
2. Ally: any...................Raise
3. Ally HP<30%.................Curaga
4. Ally: any...................Esuna
5. Self: MP<20%................Charge
6. Foe: HP<3000................Attack
7. Ally: HP<100%...............Regen
8. Ally: HP<60%................Curaga
9. Ally: any...................Haste
Third Member
1. Ally: party leader..........Raise
2. Ally: party leader..........Phoenix Down
3. Self........................Decoy
4. Self: MP<20%................Charge
5. Foe: HP<3000................Attack
Put your controller down, and go to sleep. This loop will yield roughly 108,000 EXP per hour, and runs automaticaly. Sleep for eight hours, and that's 800,000 EXP per character.