The easiest and fastest way...
Equiped an overdrive to Ap weapon and go to the Cavern of the Stolen fayth. Set your overdrive to loner. Get in a battle with a Magic Urn. Have your two other party members escape. And continually press triangle for infinite Ap.
Because...
Loner makes overdrive fill every turn you fight alone.
Overdrive to Ap makes overdrive become AP. The hardest part is costimizing for an Overdrive to AP weapon.