Since there are already great guides elsewhere online ill post a checklist and a hint or 2 about the achievement to clarify some things.
CHECKLIST
1.
https://xiii.redcrown.net/13/achieve/treasureSteam Guide Link
https://steamcommunity.com/sharedfiles/filedetails/?id=32436...Hints/Tips
1. Open every treasure ball in Chapters 1-10 and 13, even those guarded by powerful enemies
2. Do as many missions in Chapter 11 as you can! Many of them offer outstanding rewards
3. Never sell/dismantle any rings that grant elemental resistances, or any Charms
4. you don't need to have every item in your inventory at once, you just need to have held it at any point during your playthrough.
5 You only need one ultimate weapon per character.
6 The achievement doesn't unlock automatically.
7.You must talk to the robot in Oreba Village called Bhakti once you have all the weapons and accessories to unock this achievement. I was confused as to why it was not unlocking once I double checked I had all the items and such.
8. NotMitt44 says, " Make certain the language is in English. Languages like Spanish have similar names and can make it confusing when looking at the names"