I made about 20000000 potch in a few hours by fighting at the secret dungeon you can visit in the Eresh event.
The monsters there drop a lot of potch. By having Egbert, Norma (with her "Find Potch" maxxed) and 4 Prosperity Rings on every battle character, I routinely got 100000 potch per battle. Combine that with a Big Firefly Rune (to increase encounter rate--make sure to bring Jeane with you as well so you can removce the rune after you're done) and the "Dream" formation, and you can instantly kill these enemies once your party has an average level of 60 or 61.
This helped a lot in my second playthorugh.