For this, you must max out every role for every character. This will take millions of CP to achieve. While that sounds like a long, drawn out task, it's really not. If you complete mission 55 you'll be awarded with a Growth egg. This accessory gives you 2x CP for all your characters so long as it's equipped on an active party member.
Mission 55 is quite hard and can be beaten with just patience. The following setup is advised for beating it.
Party: Vanille (leader), Snow, Hope.
Also make sure you have the following Paradigms
1: Saboteur / Sentinel / Medic
2: Ravager / Sentinel / Ravager
3: Medic / Sentinel / Medic
Neochu isn't immune to Vanille's Death spell, so start off the fight with paradigm #1. Having the Malboro Wand for Vanille during this fight is almost essential. Snow, having the highest HP, will be distracting all the little Ochu's with Hope healing him. This leaves you to keep spamming Death on Neochu. It has a low probablity, but just keep trying. If your health get's too low, switch to paradigm #3, heal up then go back to #1.
Once Neochu is dead, switch between paradigms #2 and #3 as needed to finish off the small ones.
Now that you have your Growth Egg, you'll need somewhere worthwhile to train. See this thread for tips on some of the best training areas.