I can tell this app has potential. I notice this and, therefore, give it some slack on some things. I really do roll with the punches when it comes to gems AKA "the classic extremely-hard-to-earn-without-in-app-purchasing" currency. You should notice after playing this app for a day that it is accompanied by a never-ending tutorial. This actually keeps that app interesting because with most apps, once the tutorial goes away you feel free but get bored quickly. The "punch" that domes with the tutorial, though, are the "purchase" missions. At first (levels 1-10) they just seem like a bit of a nudge because they force you to spend your hard-earned coins on items you may not want yet. If you ignore the "purchase" mission you are not really allowed to work on future missions besides the Game Center ones. Once you get to around level 12, you start seeing a new problem with these "purchase" missions. The sensei keeps asking you to buy things that are not only way out of your price range, but require you to level up just to purchase without gems! In a way, it was a well-thought-out plan of the developers to create the sense of pure impatience, but unlike other apps like this you don't just "want" to get that next item faster, you are just simply repetitively told that you NEED that item or else you simply don't advance in the story. So every one of these tedious missions that seem simple are really three missions: the (implied) time-consuming level up, the (implied) time-consuming coin accumulation, and the (quick but painful) parting with your hard-earned, saved-up coins to buy an item that will satisfy one brief mission and then you never need to use that thing again.
And that was my rant.

I don't expect this to be fixed in any way.
I am actually sort of applauding the developers for creating something you can rant about and still be hooked on.