I'm on chapter 5, and it's sooo plain right now. I'm doing the same thing in each chapter basically, going straight and fighting. And there are times I can't even level up because the monsters dissaper after you beat them, is it like this in every chapter to come? Hopefully I can choose my own party members sometime and be able to actually explore and level up later on? Does anyone know

keep playing.. you will eventually be able to choose your party and grind

but the first 10 chapters of the game require no grinding at all... just kiling everything available to you should be enough to max out the crystarium levels that you have unlocked

shouldnt need to do any grinding until chapter 11

you can refight monsters by backtracking to other maps. They respawn that way.

pre chapter 11 the game is fucking horrible

Short answer to not spoil anything; No, it is not like that in every chapter.