The mechanics are pretty good and it’s overall a fun game to play. However, there’s a big flaw right now when it comes to the summoning. Most summonings cost 500 gems and most of them require several steps in order to get any good cards. In story missions and most special missions you can normally get around 5 to 10 gems the first time you complete it. There are a few, like the evolution missions that actually do give ~30 gems for completion, but once you get that, it’s gone. In the achievements, you’re lucky if a reward exceeds 5, and daily rewards are ridiculously low. So, a player might try to buy a few gems in order to get cards for their favorite character, but the cost makes it not worth it. It costs $5 for 50 gems, which can get you one (1) entire card. At $45, you can get around 800 gems, but that isn’t even enough to clear 2 rounds of normal 500 gem spawns.
I’d suggest that you increase the amount regular missions give out from 5 (low power) or 10 (medium power) to maybe 15 or 20, decrease the dollar cost to buy them, and perhaps reset the rewards in certain missions (evolution and the hero ones) every few days so high level players aren’t stuck not being able to get more gems. Alternatively, you could reduce the cost of spawns massively and get rid of multi-step ones.