The vallis media (whatever you call it) is a good place to grind CP if you really need it. Those little mechanical machines are easy to beat, respawn quickly, and offer a good amount of CP once beaten. Mark hunting, though, is ideal because it's a less tedious approach to advancing the crystarium and you get extra items along the way.

This game is pretty difficult if you're not used to the battle system. I honestly struggled on various occasions..