To get this achievement, you must obtain EVERY item in the game. This includes weapons, armor, accessories, usable items, food, materials, and special items. In other words, if it's a unique item, then you need to get it. They don't all need to be in your inventory at the same time.........simply obtaining the item once is sufficient for it to count, even if it leaves your inventory later on. This must be done on Normal or higher difficulty, because the post-game Seraphic Gate contains unique items and cannot be entered on the easiest difficulty.
I just received this achievement last night, so I can vouch that all of the resources/tips in this solution will get you the achievement if followed carefully. Here's what I used:
--jukkakhan's Item Checklist (in the form of an Excel Spreadsheet): http://www.xbox360achievements.org/forum/showthread.php?t=97...
(make sure to read the directions on how to use it correctly)
--Split Infinity's Walkthrough, covers the entire game, including side quests, treasure chests, and all missables: http://www.gamefaqs.com/xbox360/935661-infinite-undiscovery/...
--Demon9Tom's Compulsive Guide: http://www.xbox360achievements.org/forum/showthread.php?t=97...
(all of this is covered in Split Infinity's guide above, but since it specifically focuses on Compulsive, it helps keep you focused on getting all of the missables, plus it includes some handy post-game info)
--Signum7's Seraphic Gate FAQ: http://www.gamefaqs.com/xbox360/935661-infinite-undiscovery/...
(for the post-game Seraphic Gate)
--Sakurayule's Hints & Tips FAQ: http://www.gamefaqs.com/xbox360/935661-infinite-undiscovery/...
(a handy reference for locations of characters in towns, plus general tips that anyone playing through the game should know)
Basically, play through the game following Split Infinity's walkthrough, checking off items as you get them. Refer to Demon9Tom's Compulsive guide before and after entering any area as a safeguard against missables. In the post-game/Seraphic Gate, Signum7's guide is your friend.
Some tips for anyone attempting this:
--Be careful!!! This sounds obvious, but I can't stress this enough. Be organized and meticulous about keeping track of what you've gotten. If there's even ONE discrepancy between your checklist and the actual items you've gotten in game, you risk not getting the achievement and having to re-check EVERYTHING. For example, if you see a weapon that you need at a store, and then check it off on your checklist but then leave the store without actually buying it, you've already doomed yourself to not popping this achievement when you think it should. If you get an item and forget to check it off, that's OK, since you can probably get it again later, but checking off items that you don't actually have in game should be avoided at all costs!!
--Only save your checklist when you save in the game. This is extremely important, and is the best way to ensure that there's no discrepancy between your checklist and your saved game.
--Watch your pick-ups. Every area brings new enemies and new drops, so don't forget to check that upper right hand corner whenever you get a new drop. When you're fighting enemies, it's very easy to miss this.
--Use the vouchers! Without the vouchers, this achievement would probably take you 200+ hours. They can be downloaded for FREE from the XBL Marketplace, and they allow you purchase all metals, minerals, and cloths from any store you go to. It saves you countless hours of item farming.
--Save most item crafting for the end. This will ensure that you've amassed most of the items that you need during your playthrough so that you can craft most items in one go. I'd also recommend keeping a notepad file that specifies what items you weren't able to craft in your first go around, as well was what/how many materials you need to make them. This will keep your item farming much more focused and save you time!
--If you're looking to complete the game, go for Compulsive on Infinity Mode. Infinity Mode will require you to fight/level up more, which ensures that you'll get more drops, which you'll need for item crafting. Plus, beating the Ethereal Queen on Infinity will require you to obtain many of the harder/rarer items anyway (to forge the best equipment), so the two achievements go hand in hand.
--Unequip weapons/armor for them to count. If a new member joins your party with new equipment, that equipment won't count until you've unequipped them. This goes for "broken" weapons/armor as well. The last item I needed for this achievement was a "broken" weapon, and the achievement didn't pop until I had unequipped it.
--Boost your money early and buy at least one of everything at every store. Split Infinity's guide has a good strategy for making infinite money after Castle Prevant. Use it, and abuse it.
--Re-check the stores every time you go back to a town. Store inventories often change after story events, so every time you go back to a town, double check to see if they're carrying anything new. Split Infinity's guide is good about giving you a heads up when stores have new wares, but make sure to be in this habit regardless. Some items are only available at stores temporarily, and once they're inventory changes, they're gone forever.
--Use the "New Items" menu. In your inventory, you can check your "new items" tab to see what you've recently acquired. I made a habit of checking this every time I saved, to make sure that my items matched.
This achievement isn't necessarily hard, but it will test your ability to be patient and meticulous. As long as you're careful, you'll pop the achievement on your last item. Good luck!