IMO the best way to farm CP is - by far - killing the KB + Megistotherios over and over. The fight iss about 7-11 seconds, nets you 13200 CP with the Growth Egg, you have a forced pre-emptive strike and due to the fact they're fighting each other, they both start with 50% HP. I usually have ComComCom active to kill them as fast as possible. Though if you fail to kill the KB before he transforms, you probably want to have a ravager to stagger him. And you can quickly respawn the two by running back a bit behind the save point and returning. Damn quick.

Though I don't think it makes sense to level more than the 3 main jobs to Level 5 this way, for you'll need to farm Adaman(tor)toises anyway, and they also give a load of CP upon kill.

I personally use Fang, Vanille and Hope for Adamantoises and start with a SynSynMed paradigm (yeah, I leveled non-base jobs) and cast the improved Protect as well as Haste on everyone. Next I just stagger the legs and use Highwind on 'em afterwards, instantly killing them. I use SabSabSab once the Adamantoise crumbles and then go all out. Probably an unefficient way, though, as it requires killing the legs twice to finally beat it. And then you just pray for Platinum Ignot or Trapezohedron...