For this achievement, you'll need to have all roles for all characters at level 5. This requires a LOT of CP (in the tens of millions), since non-innate roles for your characters sometimes cost as much as 120,000 CP for a single node.
There's plenty of back and forth discussion as to where the best place is to grind. There are a lot of variables, which depend on the strength of your party, as well as ease of access to the spot, ease of battle, proximity to save points and chocobos, etc. So what I will do is list three prominent methods for post-game CP grinding as well as some information about them, and then you can decide which is best for you. If you are going for the Treasure Hoarder achievement, and have a plenty strong party already, I will point out a better grinding spot later that will also help tremendously in gaining the gil and items you need to upgrade your weapons.
Spot #1: King Behemoth vs. Mega-whatever method
You can find this in the northern section of the Archylyte Steppe, near the path leading to Mah'haraba. You'll see a King Behemoth and Megisthoterian duking it out. This is always a pre-emptive strike, so if you're strong enough to take out the King Behemoth before he stands up and heals himself, then this battle is a cinch. You can find a video if it here, along with a map of the location:
-My average CP per second: 129.9 CP/sec
-Guaranteed pre-emptive strike makes this a very low-risk battle
-Very easy to get to, and close to a warp stone, a save point, and a chocobo (eventually)
-A great beginner's CP grinding spot, as you can attempt this with a relatively weaker party, even before you've beaten the game.
-Recommended set-up: Lightning, Snow, and Fang as RAV/COM/COM when you're starting out (to stagger the King Behemoth). Eventually you'll get strong enough that you don't need to stagger him to kill him, at which point COM/COM/COM becomes best.
Spot #2: The Western Basin Rangda method
For directions on how to do this, check here: http://www.gamefaqs.com/boards/genmessage.php?board=950899&a...
-My average CP per second: 99.8 CP/sec
-Moderately easy to access, not close to any save points, chocobos, or warp stones
-This isn't my personal favorite, but it could work for you depending on your party strength.
-Recommended set-up: Lightning, Fang, and Snow as COM/COM/COM. Use blitz often, and make sure to target the guys that are summoning, as they can bring a King Behemoth into battle
Spot #3: The Cryohedron x 6 method
To access this, you need to gain access to a secret section of the Mah'haraba caverns. To do this, go back to Mah'haraba and ride Atomos (the big spiky ball Fal'cie) to Sulyya Springs, then ride him back. He'll have parked in a spot that now allows you to jump off of him and access the new part of the tunnels.
Enter this new area and head forward. You'll come a fork where you can jump off to the right..... but don't. Stay left. You'll come to another fork....stay left again. After this last left you'll eventually come to a mechanical tunnel. Kill the dancing birds at the end of this tunnel, then head up the stairs. Here, 6 cryohedrons will come out of the gates. Fight them for 7,140 CP, then go back down the stairs and take a few steps into the mechanical tunnel. Go back to where the cryohedrons were, and they should have spawned again. Rinse and repeat.
-My average CP per second: 145.7 CP/sec
-Moderate to poor access to save point and warpstone
-Heavily depends on having a stronger party in order to wipe them all out quickly
-This is my personal favorite spot. I'd recommend starting first at Spot #1 to strengthen your party a bit, then coming here. Once you can consistently take out the cryohedrons in 17 seconds or less, this becomes the best spot.
-Recommended set-up: Lightning, Fang, and Snow as COM/COM/COM (use lots of blitz!), or Lightning, Fang, and Hope as RAV/COM/RAV (spam fira spells), either works.
I'd HIGHLY suggest testing each of these spots for yourself, because, as I said, results will differ for everyone depending on party strength.
Now that you've settled on a spot and gotten your grind on, I recommend grinding until you have at least 2 of every character's 3 innate roles maxed out. You'll then want to obtain the growth egg as early as possible, which doubles your CP. You can obtain this from Mission 55, from a little bastard called Neochu. Ok, I lied..........he's pretty big, and he's among the hardest and nastiest bosses in the game, but there's a neat trick to beating him which you should be able to pull off at your current level (2 out of 3 innate roles maxed). Set Vanille as party leader, then pick a good Sentinel and Medic (I used Snow and Hope, respectively). Set up a SAB/SEN/MED class as your starting paradigm, then some offensive paradigm of your choice with a medic in them (Ravager/Commando/Medic, for example), as well as an emergency Combat Clinic paradigm (MED/COM/MED). You'll want to equip the Belladonna Wand on Vanille. If you can upgrade it to a Marlboro Wand, great (requires a Mnar Stone), if not, just leave it, either will work. As Vanille, just spam death over and over and over on Neochu. All of the little Pikochu's should target Snow when he provokes them, and Hope will heal Snow as needed. This means that you only need to worry about Neochu's group attacks. They're quite powerful and inflict some nasty status effects, and it's almost certain that you'll die once he uses them. But, before that happens, you should be able to get off a good number of Deaths, and eventually it'll hit. It took me about 7 tries, but it will happen eventually. Once that happens and Neochu bites the dust, (carefully!) switch to a more offensive class when you're fully healed, and take out the Pikochu's one by one. Don't push it here....these little buggers are still pretty powerful, so make sure to ALWAYS have a Medic active in this last phase, and when need be, switch to your Combat Clinic. Once you've taken them out, the growth egg is yours, and chances are you've five star-ed the mission as well! Congrats! Now equip that sucker and get back to leveling at your spot of choice (at this strength, I recommend Spot #3, but it's up to you).
Once you've maxed out all 3 innate roles for all characters, you can now attempt the Adamantortoise battles. They're in the family of mammoth dinosaur looking things that you (probably) have been avoiding while wandering about Gran Pulse. They give 40,000 CP per battle (80,000 with growth egg), AND AND AND they also drop Platinum Ingots as their regular drops (sell for 150,000 gil), and Trapezohedrons as their rare drop (need 6 to upgrade all weapons). If you're going for the Treasure Hunter achievement, this is extremely useful since you'll need a lot of gil as well as trapezohedrons to upgrade all of your weapons (required for the achievement). I will refer to this as Spot #4, only to be attempted by the very strong. I recommend all three innate roles maxed, at minimum, though more than this will certainly help. You will also need to upgrade Vanille's Belladonna Wand to a Marlboro Wand, if you haven't already, and you will need a maxed out Collector's Catalog (increases rate of regular drops) and Connosieur's Catalog (increases rate of rate drops, get this by upgrading a Collector's Catalog). If you're not familiar with upgrading, go here: http://www.gamefaqs.com/boards/genmessage.php?board=928790&a...
. You only need one of each, the bonuses do not stack. Additionally, equip accessories that boost HP, protect against physical damage, and reduce earth magic.
Spot #4: Adamantortoises
There are two of them in Gran Pulse. The best one for farming is located in the "Eastern Tors" section of the Archylyte Steppe. You'll notice that there are two paths that encircle a large, inaccessible middle section. One of these paths has a save point at the beginning of it. The adamantortoise you want is located at this path (if this description is still confusing, leave a comment and I'll try to clarify). Create a party of Vanille, Snow, and Hope, and a starting paradigm of SAB/SEN/MED, as well as an emergency paradigm of MED/SEN/MED. Now approach the adamantortoise (assuming you've made the preparations I've listed above).
If your set-up sounds familiar, it's because you're using the exact same strategy as Neochu. Spam death with Vanille. Hope should be able to keep up with adamantortoise's stomps.
The only dangers here are the Quake and Roar attacks. Every so often, a stomp will be followed by a quake spell. Your party SHOULD be strong enough to withstand this double-whammy, although you'll probably be in the red (if not, level up some more until you can survive it). Whenever he casts Quake, immediately switch over to MED/COM/MED and begin spamming Curaja's until you are mostly healed. Then switch back to SAB/SEN/MED and continue spamming death. It might take a bit, but the death will eventually get him. Once you win, head back to the save point, save, then quit out and reload. Adamantortoise will have respawned and you can do it all over again.
In the first fight that you win, take notice of the Target Time and divide that by half. In subsequent battles, if the battles goes longer than that time, just retry the battle. This is because you need five stars to get a decent shot at platinum ingots and trapezohedrons, so any battles that don't result in 5 stars are a waste of your time.
There is an alternative strategy for Adamantortoises (or any similar monster) that involves using shrouds and summoning Hecatoncheir at the beginning of the battle (which will automatically disable his legs), then going to town on him. I tried this and personally didn't like it, because each summon takes 3 TP, so the amount of time spent fighting more battles to regain TP was a lot less efficient than just spamming Death. With the Marlboro Wand and a decent magic stat for Vanille (1500+), I find that Death will most often have succeeded within 6 minutes. Sometimes it won't (at which point you should restart anyway because you've lost your five stars), but either way this is faster than the summon method. But, feel free to try it.
Once you're consistently taking out Adamantortoises, the 80k CP per battle should have you well on your way to getting this achievement, as well as the Treasure Hunter achievement as well. It's a long process, so I'd recommend having a laptop or TV handy to occupy your attention. Good luck!