Since Green Chocobo is a Medic, I recommend you build him in such a way that he not only has decent magic (in order to heal as much damage as possible), but decent HP as well, since having your Medic dying on you a lot would be a big problem, and defeats the purpose of using one. Therefore, I would go with potent materials rather than mana materials or vitality materials. Lvl 1 & 2 potent materials are best to just buy from Chocolina, since they're cheap. For lvl 3, 4, and 5 materials (potent orbs, potent essences, and potent crystals respectively) , you can farm them all effectively in the Archylte Steppe. Potent orbs are particularly easy to farm. You can get those from Buccaboos and Buccaboo Ace's in sunny weather near the gravestone/cliff area (the Red Chocobo spawns a lot her as well). They both have a 30% drop-rate, and often spawn in groups of 4-5. For potent essences and crystals, change the weather to rainy, and use the waypoint stone (cactus) to teleport to the Clearwater Marshes. Once there, you can get essences from Hedge Frogs, Mud Frogs, Swampmonks, and Caterchipillars. All of them have a 15% drop-rate, except the Swampmonks, which have a 20% drop-rate. Potent orbs can farmed in the same area from Microchus, which have a 30% drop-rate. They spawn in large numbers as well, so farming them should be easy. To be honest the essences are by far the hardest to farm, even though more types of enemies drop them.
Refer to this guide if you need any more info: http://www.gamefaqs.com/boards/619315-final-fantasy-xiii-2/61843950