You must earn 99,999,999 fol and max out the counter.
Easiest to do in post-game, in the Seraphic gate. Head to the fourth floor (Luce Plains). On the far side of the rock formation in the middle of the map is a cracked rock with a Gold Mimic Chest behind it. Killing this nets you ~60,000 fol.
From there, head to the 5th floor (Bihar Mines). Before the first elevator, there is a door to the right with another Gold Chest Mimic. He's worth another ~60,000.
From here, you can return to the fourth floor, and the first mimic will be there again. Repeat the process as much as you like.
If you bring along a lot of Gold Metal and Steel Metal, you can enchant Fol x3 / No Exp. This will get you ~360,000 fol in about 2-3 minutes of game time. Just bear in mind that you're not getting experience when farming this way.
Another good place for this is on the Castle Prevant level of the Seraphic Gate. At the end of the level, just before moving on to the next floor, is another Gold Chest Mimic. You can take this guy out, move on to the next floor, return to the Castle Prevant floor, and repeat. Utilize the Fol x3 / No Exp enchantment for big money.
However, keep in mind that this chest is way deep into the Seraphic Gate, about 15-20 minutes from the save point halfway through. Make sure you're at a suitable level for this, because there's no real "get out quick" method should things start getting rough.
Thanks to Crimson Drifter for the tip about the Castle Prevant Mimic.