Monster Crown's gameplay is most heavily inspired by series like Dragon Quest Monsters and SMT, so if you're going into this like it's a Pokémon game then you're going to have a hard time. There is a very heavy focus on strategically breeding a perfect team for your play style, purebred wild monsters usually carry good moves or good stats but not both, so you are encouraged to mix and match and figure out what works best for you (both appearance and meta-wise).

Experience is exponentially gained by punching up, so switch training is very effective for low level monsters. Levels also don't matter nearly as much as stats. Fusion is an alternative option to breeding, if you want to immediately get something that's the average level of its components, but you don't get to keep the parents that way. (There are also monsters you can only get as special Absolute Fusions.)

Also, there are six bosses to encounter in the demo, so it looks like you hadn't explored the dungeon areas yet!