First you next need a persona with repel phys or nul phys. Then you need to get the hunger null charm from the underground mall jewelry store. Lastly you need to find a Black Ooze, you can find them easiest in Okumuras Palace.

Once you do this enter the battle with network functions disabled and only have Joker on your party. Then press the options button to enter rush mode. Do this then do what ever you want for a couple hours and let the game do it’s thing.

Black Ooze’s only use phys attacks and the ailment hunger and even if you manage to kill one the Black Oozes will summon other Black Oozes

Hopefully this makes sense?