This game is a lot more fun than I expected. The IAP isn't needed at all. It's quite nice how your monsters don't gain experience themselves, meaning you are free to unsummon them to replace them with new ones at any time, without having to worry about time spent invested in the monster.
I really hope they didn't pay much for the translation though - 'Frying Manta', 'Cray Golem', and 'Gorgon' - which is a stone bull. I'm not sure what they were trying to call the bull, but I'm pretty sure they didn't mean gorgon.
Also, lol @ the art. Levelling up = less armour. I'm embarrassed to play this on the train to work!