You need a PSX control with a Turbo button
You should place your PSX in a cool place because running the entire night can burn it!
How is possible making the characters training automatically?
Easy. Go to the Midgar Zolow Cobra place. This snake you know so well is extremely powerful although it doesn't give up that much EXP and AP... But it is an enemy that never stops coming automatically for you!
Equip all the characters with Death Blow + HP Absorb and Death Blow + Counter. This way when the Cobra attacks you, you counter with a powerful blow that not only inflicts massive damage but it will get 10% in HP from that damage.
Place all your characters right in the middle of the lake. Turn on the Turbo button and wait for the cobra. One she reaches you, notice you will attack normally and counter attack with the Death Blow.
Since you get 10% of the HP from the counter attack your HP will never lower so you can leave your PSX on for endless hours...
You can level up to 99 but you will need at least one week day and night!
