Once you reach Chapter 11 you can start to find some ideal leveling spots to accelerate though those CP grinding.
Reached Chapter 11 and above.
To help this even more I suggest you get the Growth Egg as this will double all CP earned. Make sure to read through the Tactics for each spot for additional preperations.
Start from #1 and work your way up to #3 changing only when you feel comfortable.
In this spot you will be focusing on Behemoth Kings fighting alongside Triffids and can be attempted from Chapter 11 onwards (good spot for weaker parties).
In this spot you will be farming CP from the Behemoth King and Megistotherian that are fighting each other close to the path leading to Mah'habara (best location pre-post game).
After completing the main storyline, the Adamantoise offers some of the best rewards in the entire game: 40,000 CP, a Platinum Ingot as its normal drop (sells for 150,000 Gil), and a Trapezohedron as its rare drop. The later of which is essential to create ultimate weapons and costs 2,000,000 Gil to buy otherwise.