the only ill noticeable thing I have found about the battlfields is that If I say: Go to the first battlfield accesible after finding the first battlefield key. I can only access one entrance even though I can browse the whole map and once I have killed all the enemies than I am limited on the amount of experience I can rack up.

I have not tested this theory yet but im assuming that since they are randomly generated to an extent you could always exit the battlefield and re enter it to respawn so to speak the additional monsters you may or may not need for the extra experience.