One of the most frustrating aspects of Final Fantasy XIII for completionists gamers out there is that there are several missable items and Datalog Enemy Intel entries that it’s easy to overlook if you don’t know what you’re looking for. Even more frustrating is that several of the missable items are required in order to obtain the Treasure Hunter trophy.
Are you trying to obtain all of the Achievements / Trophies in the game? If so, you can use this guide to help you make sure you don’t skip past any missable items. Note that only the weapons and accessories are required for the Treasure Hunter achievement / trophy. Other Missable content, including scenes and Enemy Intel entries, will not impact gameplay in any way.
Note that there are several places to get many of the Missable Content items (
Spark Rings, Ember Rings, 
Doctor’s Code, etc.). If you missed one of these items you can still pick up any of the others in later chapters and be okay for the Treasure Hunter trophy, but these items are still mentioned here because they tend to be overlooked the most often by players.
All of the missable items have been flagged in the walkthrough and strategy guide sections to help make sure that you don’t miss any of them.
Elixir