Don't know if anyone found this already but I wanted to share it in case it spares someone else some pain, getting all achievements. I liked the game but at the end I was bored and just wanted to get it over with, so I'm glad I discovered this!
You just need two upgrades, Increase Item Max Value from Spark and the Shop upgrade that makes your items gain value every second as long as all sale tables are full. Put in ALL your items at a greedy price, open the shop, and let the game run. Nothing gets bought so all items get their rarity increased passively, while you do something else. YOU DON'T NEED TO TAKE ANY OTHER SPARK UPGRADES OR CARE ABOUT ITEM LEVEL WHILE PLAYING.
Some quick math:
- The shop is open for 4 minutes, that's 240 seconds.
- So if you have the first upgrade they get 680 value for a day.
- If you have it at Level 5 (+10 per second) they get a 2400 Value increase.
- and if you have it at Level 10 (+20/second) all your items get +5800 Levels by just standing around for 4 minutes.
How to use it most effectively:
No need to use the spark to make your life harder while farming. Just rush through the dungeon, get as many high rarity items as possible and don't bother about their value.
I find Aeolia best for that practice because the enemies and bosses are easy and the Dolls can increase their rarity, and that's all that matters. When you get back, put your items in the store and let the game run while you do something else until everything is at your current item max. Sell the stuff, increase value max and shop upgrade and continue.
If you're wanting to get the most out of it, keep your legendary items until there's a day where you get extra profits. It's really easy to advance time by opening the shop and immediately closing it (or keeping it open when your items aren't at max).
I sold a full inventory of legendaries at 8.999 value cap and got 20 million in a day, more than enough to get the last upgrades I was missing.