OK So which enemy has the Trapezohedron? The Adamantoise or Adamantortoise? How can I tell which one is which, are they in certain areas or something? I'm able to kill some using Death, it's just annoying to have to go back to the main menu every time I kill them all.

The giant ones..both kinds i believe
 
also you should try to make Nirvana or Kain's lance (upgraded from pandoran spear) and then dismantle that bitch for 3 trapezohedrons!

" The giant ones..both kinds i believe  also you should try to make Nirvana or Kain's lance (upgraded from pandoran spear) and then dismantle that bitch for 3 trapezohedrons! "

Trapezohedrons are only worth around 12k if I remember correctly- not 1 million.  The Adamantoise/Adamantortoise both have Trapezohedrons being a Rare drop and Platinum Ingots being the common drop.  Selling Platinum Ingots will net you 150,000 gil so it's wise to sell that instead.

Hook up your characters with that Collector Catalog or Connoisseur Catalog. One of them make the Common drop rate better and the other goes for Rare drop.

Remember it takes 1.5million gil to upgrade an ultimate weapon to max before you can dismantle it into 3. There's no point in having 3 ult weapons if they are all low level, it's much more useful to have a fully upgraded Kain's Lance first, and farm the last 3mil through ingots. By the time you have the gil needed, you will easily have 4-5 Trapezohedron.
 

There is no quick and easy way to get that treasure hunter achievement. Completing it will take 10+ hours of grinding due to the low drop rate of the ingots and traps. Upgrading all the other accessories also costs quite a bit. 

" @Vorbis: I've been farming for over 5 hours and I don't have a single Trap :( "

I finally got one guys!! I took away the Collector's Catalog and I got one about an hour later. Oh well, at least I got around 150,000 from the Ingots. 
 
I have another question though - for the collect all weapons and accessories achievement, Do I only need to upgrade to one ultimate weapon for each character, or every weapon's ultimate form even though they're called the same thing?

@xyzygy
I believe you only need one ultimate weapon each.
 
It was explained to me as one of each name, so items with the same name don't count.  I don't have the achievement yet, but that's what I know so far.

Damn... I wish there was a faster way with this. All day today I farmed and only got one. And not only that the Connosieur Catalog doesn't work with the Collector's Catalog so I have to decide between waiting it out for these stupids Traps or getting shit tons of money.... It's seriously stressing me out but I want these things so bad. 
 
On another note, maxed out all characters while farming. Hooray.

You can also get a trap by dismantling a maxed out genji glove. You can get 3 of them in the game from missions 51, 62, and 63. 

Damn, I didn't know you had to unequip the collector's catalog. Sometimes the strategy guide is not that helpful. I'm gonna try that whole Kain's Lance strategy. But use it on Fang's cheaper weapon.

Right now I'm done with the whole Death thing with Vanille. It's so annoying and takes so long, just killing them with Fang's ultimate weapon is a lot more efficient, I can usually kill them in about 2 minutes. I can't wait until I get another Trapezo so I don't have to summon... let me just say I'm glad SE gave an option to skip the summon cinematics.

Now if only they had a skip feature for Sephiroth's Meteor 'I'm gonna mess up the Galaxy and burn you with the sun' move...

What are you guys saying? I'm pretty sure the Collector's Catalog and Connoisseur's Catalog stack together. That's what the official guide says. I've been having them both equipped and they seem to both work.

" You can also get a trap by dismantling a maxed out genji glove. You can get 3 of them in the game from missions 51, 62, and 63.  "

" What are you guys saying? I'm pretty sure the Collector's Catalog and Connoisseur's Catalog stack together. That's what the official guide says. I've been having them both equipped and they seem to both work. "

Wow. I can confirm that, after getting the Long Gui on the Archylte Steppe (finally), the drop rates for the Traps are higher. I just got 2 in a row!

I read that monsters only drop one item. I guess equiping both the Collector and Connoisseur isn't to our advantage.

" I read that monsters only drop one item. I guess equiping both the Collector and Connoisseur isn't to our advantage. "

So now I'm getting more Trapezohedron drops than Platinum Ingots! -_- 
 
Does anyone know of a better way to make money than getting Platinum Ingots? I haven't gotten one in ages.

Two of the same catalog don't stack. 
Different catalogs can work simultaneously, but that isn't stacking, because their effects are different. 
 
I have all 3 catalogs on my party, and I've had battles than ended with all 3 drops, shrouds, rares and normals all at the same time, so I know they work together.

Get a Kain's Lance (made from the Dragoon Lance, Shamanic Spear, Punisher, Pandoran Spear or Gale Bolg). Upgrade the Kain's Lance to maximum level (at a cost of approximately 1,5000,000 gil) and then dismantle it. You'll Obtain three Trapezohedrons, tripling your initial investment. You will also get 36 Moonblossom Seeds at the same time, worth 216,000 Gil. You can do this with a Nirvana (made from the Tigerclaw, Healer's Staff, Belladona Wand, or Mistilteinn) as well.