How tf am I supposed to farm 40k yen on 22/4 and why do we even need that money

Was following this guide: https://gamefaqs.gamespot.com/ps5/410283-persona-3-reload/faqs/81159/april

I didn't even notice that I spend like 2-3 hours in this part of the dungeon and I ain't even close.

And how do you even farm rare shadows at this stage ? Is there a RNG trick?

Need to grind a little more to get Yukari to 11