Having Triple AP, Triple Overdrive and Overdrive -> AP generates enormous amounts of AP.

Fight Don Tonberry with all chars equiped with a weapon with those skills.

Have one, and only one char attack the Don, and set his Overdrive Mode to Stoic.

Set the others Overdrive Modes to.... ally?, anyway, the one that fills be Overdrive bar when an ally gets hit.

Whn the attacking char hits the Don, he (the Don) will counterattack with Everyones Grudge, and there is a high probability that the attack will deal 99.999 points of damage.

Revive the downed char and repeat until the Don bites the dust and you will get 99 S.Grid Levels.