From what I have found through searching these forums and testing myself, is that 2-1 is the best stage to farm until 12-3 and then 12-5. It seems strange, but the game has a rather linear reward scale based on stamina cost, with only some slight variation, and 2-1 is one of the best in terms of stamina to reward. The special stages are the exception, most of them have much higher stamina cost than the reward they offer, which makes them not worth doing. The 12 cost hidden stage though is very good to do, which is the other reason to farm 2-1. So you grind 2-1, do the hidden stage every time it comes up, and sell all the 2 stars you get from the stage and from the gacha.

Quote Originally Posted by Discoceris View Post
-Target leveling of high level characters in one go. You waste a LOT of coin leveling them one high level manyu at a time. As those girls get higher in level, the cost for leveling them goes up for each manyu/exp slot you fill.

Example: instead of leveling a girl 2,500 + 3,000 + 3,500 + 4,000 + 4,500 (17,500), it is much more preferable to level her 2,500 x 5 (12,500) You may think saving 5K in this instance not a whole lot, but let me tell you about human behavior. If you engage in the former, you will do that repeatedly, over and over, which results in hundreds of THOUSANDS in wasted coin. If you practice the latter ritually, you end up saving lots of coin, and not seeing the same kinds of problems other players experience.
Gradual increases in power is what allows one to progress in the game though. Also the cost increases only about 70 gold per level for non-evolved girls and 100 for evolved, so the difference in upgrading with 5-10 manyus compared to doing them 1-2 at a time is only a few hundred gold. It's honestly not worth thinking about IMO.