Levels aren’t enough to win your battles in Final Fantasy XVI; you need abilities. Collecting ability points (AP) to master them is tricky—so tricky that you can beat the game without even mastering the abilities you use. This farming guide can help you save precious time.
Final Fantasy 16 – Best AP & XP Farms Early, Mid & End Game – 60K AP Per HOUR Tips Final Fantasy XVI
Actually, farming AP in FF16 isn’t that hard. All you need is an accessory called The Wages of Warcraft and a stage map with a bunch of monsters to kill. The Wages of Warcraft is an accessory that allows you to earn 20% more AP in normal or mob battles. I don’t recommend grinding AP without this accessory, but you can only get it after a certain point in the story.
You can purchase The Wages of Warcraft from Lady Charon (Cid's Hideaway) after the five year time skip in the game. If you're not in a hurry, farm for AP before the point of no return, where you'll have access to better areas and can overpower mobs at your level.
Then make sure you have all the AoE/area-wide abilities you need. This will help you defeat enemies faster. Some examples are Ramuh's Pile Drive, Titan's Upheaval and Phoenix's Flames of Rebirth. If you bought the FF16 DLCs, then only Leviathan's and the second Eikon's special ability will be enough.