Asking demons for money and the scout app that gives you money whenever you recruit demons + bonus demon recruited are the best way of getting money (I was getting about 5K macca whenever I managed to recruit two demons in one go during the end game) but that would still take a while to get the stuff from the sikrit shop. I didn't bother with it on the first playthrough because it'll be an easy get on New Game + : you'll find yourself spending a lot less money in new game+, so by the time you reach that point again, you should have enough to splurge to your heart content.
Also keep selling the item drops. Things like the end game MP replenishment items and revival stuff can be worth up to 5K each and they're not that necessary for combat if you focused your app unlocks on Expand Stock. Actually, I think Expand Stock combined with Summon Discount are the most powerful apps in the game once you're done extending your skill slots. Because demons in your stock also get XP it nets you a lot of level up gifts and the like, potential skill mutations fodder for fusion and you never run out of healing because you always have an army of demons with you. I always had something like 5 demons with media and mediarahan, recam and posumudi.
Also keep selling the item drops. Things like the end game MP replenishment items and revival stuff can be worth up to 5K each and they're not that necessary for combat if you focused your app unlocks on Expand Stock. Actually, I think Expand Stock combined with Summon Discount are the most powerful apps in the game once you're done extending your skill slots. Because demons in your stock also get XP it nets you a lot of level up gifts and the like, potential skill mutations fodder for fusion and you never run out of healing because you always have an army of demons with you. I always had something like 5 demons with media and mediarahan, recam and posumudi.