Raise Noel and Serah to Level 99 in all 6 Roles (Commando, Ravager, Sentinel, Saboteur, Synergist, Medic) in the Crystarium.
As in the first Final Fantasy XIII, you can earn 2x CP with "Rolling in CP" (a Fragment Skill this time, not an Accessory). Rolling in CP can be unlocked via the Chocobo Girl in Serendipity after all Fragments from Bresha Ruins AF005, AF100, and AF300, as well as all Fragments from Yaschas Massif AF010, AF100, AF110, and AF01X have been earned.
Another method to earn more CP is via monster abilities. There are a few monster abilities that can increase CP - One that comes to mind is a Level 99 Green Chocobo for its last ability, "CP Plus." Green Chocobos are also decent healers and can even be used in Chocobo Racing, so it is a monster worth investing into.
Compared to Final Fantasy XIII, Supreme Being shouldn't require a lot of grind time due to the addition of CP awarded from Fragment Missions. Defeating Ochu in Archylte Steppe (one of the monsters required for Big Game Hunter and a Fragment) rewards 30,000 CP. Paradox Endings often reward 10,000+ CP. There are a lot of opportunities to earn a lot of CP, and around the time I finished off the , Noel and Sarah both had over 400,000 UNUSED CP at their disposal due to the fact that I had nothing left to allocate it to.