This is my monster box.
I'm finding myself lost, I don't know which monsters to level up and which monsters to use to level up those monsters xD
Any suggestion?
Tyrra and Isis (the two monsters in the left corner) are the monsters you should work on, in particularly Isis because her leader skill can help you clear a large portion of the content in the game. Everything else in your box are just place holder monsters that you can use to fill out your team, food for Isis and Tyrra, or evolution material for Isis and Tyrra.
However hold on to the two Tamadras (the little gold egg monsters). Those are used to awaken skills on monsters in their final/ultimate evolutions and are very hard to get, so save them until later in the game.