The Last Remnant

You need to create the Ragna-Rock at the normal equipment maker in any town.

You need 888,888 cash, Twilight of The Gods, A Crystal Scull and a Clay Dinosaur Idol.

You get Twilight of the Gods after completing the quest "The Fallen" (see The Godslayer achievement). You can also get one by defeating Demigod at the top of the Ancient Ruins (requires free DLC). I'm not sure if Demigod is re-spawnable or not or if the Twilight is a guaranteed drop, so best to save just before facing him so you can reload in case he doesn't drop one. Although you won't want to fight him more than once :-/

You get the Clay Dinosaur Idol after beating the Beserk Cyclops as part of the main story after the 6 base battles. You can also get one by beating Cyclops Standard Model at the bottom of Avecliff (require free DLC). Note that the Cyclops Standard Model is not re-spawnable and the dinosaur idol isn't a guaranteed drop, so save just before fighting him and reload if it doesn't drop one when you kill it.

The Crystal scull you get when you defeat Numal Niram as part of the side-quest "The Tablet of Marshall". There's no other way to get one as far as I know.

Note that these can all be used to create other great equipment so be careful not to use them up before you've got the achievement. The Ragna-rock is not really worth the cash, so save before creating it and then reload :) Then you can create the better equipment with these components and keep your money.

On the subject of money... if you complete all side-quests you'll get 1,000,000 which is what I used to create it. If you've not done all the side-quests then it'll probably take a lot of boring hours to raise the cash :-/

I'm not sure where the recipe comes from. Perhaps from beating The Fallen? Or maybe it just appears when you have the ingedients.