Here are some things to keep in mind.
1. Big nodes on crystarium increase stats in a certain way.
Commando - Increases str the most.
Ravenger - increases mag the most.
Sentinel - increases HP the most.
Sabatuar - increases str even mag on odd like 1, 3, 5, 7 str and 2, 4, 6, 8 mag.
Syn - increases hp and mag like sabatuar.
Medic - increases HP
Try to max out str for noel and mag for sereh.
2. Certain monsters are worth farming right away.
Green Chocobo - HP +35%
Tonberry - str and mag up 35%
You might want to look at a guide for important monsters.
3. Know almost everything is re-doable after the final boss.
You don't have to worry if you miss stuff cause most the game is repeatable.